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Evaluating how clear the questions being investigated in randomised trials are: systematic review of estimands

BMJ 2022; 378 doi: (Published 23 August 2022) Cite this as: BMJ 2022;378:e070146

Linked Opinion

Time to improve the clarity of clinical trial reports by including estimands

  1. Suzie Cro, advanced research fellow1,
  2. Brennan C Kahan, senior research fellow2,
  3. Sunita Rehal, principal statistician3,
  4. Anca Chis Ster, doctoral student4,
  5. James R Carpenter, professor of medical statistics2 5,
  6. Ian R White, professor of statistical methods for medicine2,
  7. Victoria R Cornelius, reader in medical statistics1
  1. 1Imperial Clinical Trials Unit, School of Public Health, Imperial College London, London, UK
  2. 2Medical Research Council Clinical Trials Unit at University College London, London, UK
  3. 3GlaxoSmithKline, London, UK
  4. 4Kings College London, London, UK
  5. 5London School of Hygiene and Tropical Medicine, London, UK
  6. Correspondence to: S Cro s.cro{at} (or @Suzie_cro on Twitter)
  • Accepted 21 June 2022


Objectives To evaluate how often the precise research question being addressed about an intervention (the estimand) is stated or can be determined from reported methods, and to identify what types of questions are being investigated in phase 2-4 randomised trials.

Design Systematic review of the clarity of research questions being investigated in randomised trials in 2020 in six leading general medical journals.

Data source PubMed search in February 2021.

Eligibility criteria for selecting studies Phase 2-4 randomised trials, with no restrictions on medical conditions or interventions. Cluster randomised, crossover, non-inferiority, and equivalence trials were excluded.

Main outcome measures Number of trials that stated the precise primary question being addressed about an intervention (ie, the primary estimand), or for which the primary estimand could be determined unambiguously from the reported methods using statistical knowledge. Strategies used to handle post-randomisation events that affect the interpretation or existence of patient outcomes, such as intervention discontinuations or uses of additional drug treatments (known as intercurrent events), and the corresponding types of questions being investigated.

Results 255 eligible randomised trials were identified. No trials clearly stated all the attributes of the estimand. In 117 (46%) of 255 trials, the primary estimand could be determined from the reported methods. Intercurrent events were reported in 242 (95%) of 255 trials; but the handling of these could only be determined in 125 (49%) of 255 trials. Most trials that provided this information considered the occurrence of intercurrent events as irrelevant in the calculation of the treatment effect and assessed the effect of the intervention regardless (96/125, 77%)—that is, they used a treatment policy strategy. Four (4%) of 99 trials with treatment non-adherence owing to adverse events estimated the treatment effect in a hypothetical setting (ie, the effect as if participants continued treatment despite adverse events), and 19 (79%) of 24 trials where some patients died estimated the treatment effect in a hypothetical setting (ie, the effect as if participants did not die).

Conclusions The precise research question being investigated in most trials is unclear, mainly because of a lack of clarity on the approach to handling intercurrent events. Clear reporting of estimands is necessary in trial reports so that all stakeholders, including clinicians, patients and policy makers, can make fully informed decisions about medical interventions.

Systematic review registration PROSPERO CRD42021238053.


The results of randomised controlled trials are used in policy making and clinical practise to make decisions about which medical interventions to use. However, informed decision making requires an understanding of the precise question being investigated in a trial, because different questions can lead to different conclusions about the usefulness of an intervention.123456789 For example, a trial in type 2 diabetes10 compared a once weekly insulin regimen with a once daily regimen on the change from baseline in glycated haemoglobin, and asked two different questions. Firstly, what was the treatment effect if all participants had hypothetically adhered to the treatment regimens and not received ancillary treatment (hypothetical effect); and secondly, what was the treatment effect regardless of the amount of randomised treatment or ancillary treatment received (treatment policy effect). The hypothetical effect was twice as large as the treatment policy effect (mean difference −0.18 percentage points (95% confidence interval −0.38 to 0.02, P=0.08) v −0.09 (−0.29 to 0.20, P=0.35)).10 Therefore, depending on which treatment effect was considered most relevant in decision making, conclusions can differ substantially.

However, the specific questions that trials investigate are not always clear, and often stems from ambiguity in how events after randomisation (eg, intervention discontinuation or use of rescue therapy; termed as intercurrent events) are handled in the definition of the treatment effect. In some cases, the relevant information is omitted, or expert statistical knowledge might be required to decipher this from reported methods. For example, a placebo controlled trial in atopic dermatitis reported baricitinib in combination with topical steroids significantly reduced impairment in daily activities.11 All randomised participants were included in the analysis in their randomised group, so readers using the results to inform decision making might assume that the trial addressed the intervention’s effect if adopted into routine practice. However, on close inspection of the statistical methods, the trial assessed the intervention effect in the hypothetical situation where participants who stopped treatment had instead continued and rescue therapy was denied. This interpretation is because investigators set outcomes recorded after discontinuation and receipt of rescue therapy to missing, and then used a statistical model that implicitly imputed what the participant’s outcome would have been had they not discontinued treatment or received rescue therapy (table 1).

Table 1

Deciphering the research question being investigated in example trial11*

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To tackle such issues and avoid trial results being misinterpreted, new international trial regulatory guidance (ICH E9(R1), the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) E9(R1) addendum on Estimands and Sensitivity Analyses in Clinical Trials, November 201912) has called for trials to precisely define the clinical questions being assessed by specifying estimands. An estimand is a precise description of the treatment effect that a trial is aiming to find out (ie, the question to be answered). It expresses what the numerical result (the estimate) represents including with respect to intercurrent events. It is entirely separate to the statistical methods (the estimator), which specifies how the trial will compute the result. An example estimand and strategies for handling intercurrent events are outlined in table 2.

Table 2

Definitions of estimands, estimators, and estimates, using an example trial13*

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Current trial reporting guidelines (CONSORT15) were established before the introduction of ICH E9(R1) and do not require trialists to specify estimands. As this area of focus is new, and while medicine regulators worldwide are adopting ICH E9(R1) guidelines, we aimed to determine current practise and establish whether the reporting of estimands in trial reports is necessary to fully understand the questions being investigated in clinical trials. In our study, we reviewed published randomised trials with the specific objectives of evaluating how often the precise question being assessed in a trial was stated or could be determined from the reported methods using statistical knowledge, and to identify what questions are being investigated.


The protocol for this systematic review is in the supplementary material and is registered on PROSPERO.16

Search strategy

We examined randomised controlled trials published in the year 2020 in six high impact general medical journals: Annals of Internal Medicine, The BMJ, Journal of the American Medical Association (JAMA), The Lancet, New England Journal of Medicine (NEJM), and PLOS Medicine. We searched in February 2021 for articles in PubMed with a publication type of “randomised controlled trial,” or including the keyword “random*” in the title or abstract, or categorised with the MeSH term “random allocation.” The full search strategy is in appendix 1 in the supplement.


Phase 2-4 randomised trials in humans were eligible for inclusion, with no restrictions on medical conditions or on interventions or comparators. Cluster randomised, crossover, non-inferiority, and equivalence trials were excluded since estimands and statistical issues within estimation might be different for these trials. Other exclusions included pilot or feasibility studies, phase 1 studies, non-randomised studies, secondary analyses of previously published trials, a primary outcome of cost effectiveness, more than one trial reported in the article (including meta-analysis and systematic reviews), interim analyses, or letters or commentaries.

Title and abstract screening of search results for eligibility was performed by one author (SC). Full texts of articles were then assessed independently by two statistical reviewers to confirm eligibility and extract data (SC, BCK, SR, or ACS).

Data extraction

Data were extracted onto a pre-piloted standardised data extraction form (see supplement). Disagreements were resolved by discussion, or a third statistical reviewer where necessary. Where the trial publication referred to supplementary material (excluding a protocol or statistical analysis plan), the extractor referred to these documents. Extracted data included: trial characteristics, occurrence of intercurrent events, and whether the primary estimand was described for the trial’s primary objective or if information on the statistical methods (estimator) or other reported methods could be used to determine unambiguously what the estimand was using statistical knowledge. We also extracted whether supplementary estimands, defined as treatment effects that handled intercurrent events in a different way for the primary outcome, were described. Data on estimand specification in protocol and statistical analysis plans were extracted separately where these documents were available in supplementary material or referenced within the main article and publicly available.


For each trial’s primary estimand, two statistical reviewers independently assessed whether each of the five estimand attributes (table 1) was explicitly stated, not explicitly stated but unambiguously inferable, or not inferable using similar methods to those used in a recent review of estimands in protocols.17 If an estimand was described as primary we used this as the primary estimand; if none or multiple estimands were listed as primary, we used the main analysis of the primary outcome to determine the primary estimand. If no analysis approach was described as the main or primary analysis, we used the first analysis approach listed in the statistical methods section for the trials’ primary outcome.

For intercurrent events, we first determined which of eight event categories referenced within the ICH E9(R1) addendum were relevant to the trial (see appendix 2 in the supplement), and then extracted data on the handling for the relevant events. In line with ICH E9(R1)12 and as described in table 2, strategies for dealing with intercurrent events were categorised as treatment policy, hypothetical, while-on-treatment, composite, or principal stratum. The overall strategy for handing intercurrent events was recorded as “stated” if the handling of all occurring intercurrent events was explicitly stated; “inferable” if the handling of all occurring events could be deduced from information in the publication or were stated; and “not inferable if one or more occurring intercurrent events handling was not inferable. Where non-treatment policy strategies were used—for example, the treatment effect for all patients with diagnosis of interest if all individuals adhered to medication, the statistical methods used for estimation were extracted.

Attributes were considered as “stated” if these were explicitly described as part of an estimand definition or if listed as part of the trial objective. For example, if the article included a description of the estimand and stated that the targeted population was all patients meeting the trial inclusion or exclusion criteria, this attribute would be classed as stated.

Attributes were “inferable” if they were not stated as part of an estimand definition or trial objective, but could be unambiguously deduced based on the statistical methods (estimator) or other reported methods using statistical knowledge. For example, if the article stated that the analysis population was all randomised participants and the analytical approach also targeted an effect for the full population, then the population attribute would be inferred as all patients meeting the trial inclusion or exclusion criteria. If an intention-to-treat analysis was specified and data collection continued after the occurrence of intercurrent events, the treatment condition could be inferred as the offer of treatment, and a treatment policy strategy could be inferred for handling non-terminal intercurrent events. If the statistical methods stated the type of summary measure that would be estimated (eg, a mean difference), or it was clear from the analysis model what the estimated measure was (eg, linear regression model), then the population level summary measure could be inferred (eg, targeted a mean difference).

Alternatively, if it was not clear how to reconstruct the estimand attribute or if more than one estimand was consistent with the reported information, attributes were “not inferable.” For example, if the analysis population excluded participants who did not receive all treatment, such as a per protocol analysis, it was unclear whether a principal stratum population of patients who would receive all treatment was of interest or whether the entire trial population meeting inclusion or exclusion criteria under a hypothetical strategy was of interest. Since both interpretations are consistent with the presented information, the population was set as “not inferable.” In this scenario, we also set intercurrent events handling to “not inferable” because it was not clear whether this corresponded to a hypothetical or principal stratum strategy.

The overall estimand was considered as “stated” if all five attributes were clearly stated, “inferable” if the five attributes were a mix of stated and inferable, or “not inferable” if one or more attributes was not inferable. We also assessed how well supplementary estimands that handled intercurrent events in a different manner to the primary estimand for the trial’s primary outcome were described. Where protocol or statistical analysis plans were available, we separately assessed whether these documents stated any estimands.

Statistical methods

Outcomes were summarised descriptively using frequencies and percentages. We performed two prespecified subgroup analyses, which summarised outcomes separately by trial sponsor (pharmaceutical or for-profit (eg, medical device companies) v academic/not-for-profit) and self-defined pragmatic trial (pragmatic v not pragmatic). Analyses were performed using Stata version 15.

Patient and public involvement

Patients and the public were involved in the initiation and interpretation of this study. The occurrence and potential impacts of post-randomisation events, such as treatment non-adherence, in clinical trials were described to members of the public (n=9, aged 20-60 years, mixed sex and ethnic groups) at a people’s research cafe run by the NIHR Imperial Patient Experience Research Centre. They supported the importance of researchers appropriately taking into account such events in clinical trials and the conduct of this research to understand how this is done. As the results of this study emerged, we reviewed selected outcomes with the public advisory panel for the HEALTHY STATS research project (NIHR300593). The public advisory panel aims to improve information reported from clinical trials for patients and health care practitioners: it includes five public partners aged 20-70 years of mixed ethnic groups and sex. The group were surprised that the type of question investigated in a clinical trial is not always clear: they highlighted the need for the specific trial question to be reported alongside the numerical results to ensure clarity.


Search results and trial characteristics

The search identified 753 articles, of which 255 were eligible randomised controlled trials (eFig 1). Most trials (175, 69%) had a drug intervention; 162 (64%) had an academic or not-for-profit sponsor; 93 (36%) had a pharmaceutical or for-profit sponsor; and the median sample size was 402 participants. Further trial characteristics are summarised in eTable 1 in the supplement.

Primary estimand

No articles completely stated the primary estimand (fig 1). Four (2%) trials attempted to explicitly state the estimand, but each of them omitted one or more attribute (table 3). The treatment effect investigated in the trial could be determined from the reported methods for 117 (46%) trials, as all estimand attributes were inferable. We were unable to determine the target population in 82 (32%) trials, treatment condition in 28 (11%) trials, handling of intercurrent events in 117 (46%) trials, and the population level summary measure in 31 (12%) trials. Reasons why attributes were stated, inferable, or could not be determined are presented in eTables 2-3 in the supplement.

Fig 1
Fig 1

Description of primary estimand reported in 255 eligible randomised controlled trials, by estimand attribute. Table 1 provides definitions of estimand attributes. IE=intercurrent events (eg, intervention discontinuation or use of rescue therapy)

Table 3

Attribute details of primary estimands in eligible randomised controlled trials

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Intercurrent events

Two hundred and forty two (95%) trials reported at least one intercurrent event that could affect the interpretation of outcome data (table 4 and fig 2). We could determine how intercurrent events were handled in 125 (49%) trials (n=4 stated strategy, n=121=inferable). Where stated or inferable, most trials used a treatment policy strategy for handling all relevant intercurrent events (96/125, 77%), meaning that they considered the outcome regardless of any intercurrent events. Hypothetical or composite strategies were used to handle at least one type of intercurrent event for 17/125 (14%) and 12/125 (10%) trials. Statistical methods used for estimation of non-treatment policy strategies are summarised in table 4. Four (4%) of 99 trials with treatment discontinuation due to an adverse event estimated the treatment effect in a hypothetical setting, if participants continued to take treatment despite adverse events, and 19 (79%) of 24 trials with deaths considered a hypothetical setting, if participants did not die. Strategies for other intercurrent events are shown in eTable 5-6 in the supplement. Subgroup analyses by sponsor type and pragmatic trial design did not reveal any notable differences (eTables 7-8 in the supplement).

Table 4

Statistical methods used for handling intercurrent events in eligible randomised controlled trials, by strategy (excluding treatment policy strategies)

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Fig 2
Fig 2

Intercurrent events occurring in eligible randomised controlled trials (n=255). Unclear if occurred=intercurrent event described in the introduction or methods but no frequency data reported, therefore was potentially an intercurrent event but not possible to ascertain whether actually occurred in the trial. *Non-adherence=treatment non-adherence or discontinuation for the given reason. †Additional=not part of usual care (eg, rescue or prohibited treatment). ‡An intercurrent event is defined as an event which occurs after randomisation and effects the existence of interpretation of trial outcomes; where death was the primary trial outcome, by definition this excludes the possibility of death being an intercurrent event. §Other terminal events observed include graft failure, termination of pregnancy, miscarriage or medical termination of pregnancy <20 weeks, pregnancy loss <22 weeks, cancelled surgery (outcome pain use within first 24 hours post-surgery). ¶Other intercurrent events are listed in eTable 4; 33 (13%) trials had one other intercurrent event, nine (4%) trials had two other intercurrent events, and two (1%) trials had three other intercurrent events

Supplementary estimands

One hundred and twelve (44%) trials used at least one supplementary estimand that handled intercurrent events in a different manner to the primary estimand. Sixty three (56%) of these trials incorrectly indicated that the supplementary estimand would deal with the same question as the primary estimand (ie, by mislabelling the supplementary estimand as a sensitivity analysis), which could cause confusion if results differ. No supplementary estimands were fully stated. One or more supplementary estimand was inferable for 28 (25%) trials including supplementary analysis (eTable 10-11 in the supplement). The handling of intercurrent events for one or more supplementary estimand was stated or inferable for 34 (30%) of 112 trials, including 28 (82%) using at least one non-treatment policy approach. Other strategies used included hypothetical, composite, or principal stratum (see table 4 for statistical methods).

Use of estimands in protocols and statistical analysis plans

For 231 (91%) 255 articles, a protocol or statistical analysis plan was available (198 supplementary material, 25 published, and eight on references website). Of these 231 trials, 18 (8%) used the term “estimand” in the protocol or statistical analysis plan, including 16 with a pharmaceutical or for-profit sponsor and two with an academic or not-for-profit sponsor. The primary estimand was defined and fully stated by trial authors for four (2%) trials and partially stated for 10 trials (4%) within the trial protocol or statistical analysis plan; the remaining four trials that used the “estimand” term did not actually define what the estimand was (eTable 12 in the supplement). The handling of at least one intercurrent event was stated for 14 trials. Comparison with the results article revealed that eight trials had intercurrent events occurring that had not been planned for within the estimand. The stated estimand attributes are summarised in eTables 13-15 in the supplement and show variability on what is being stated.


Principal findings

For over half of the 255 trials in this study, the precise primary question being investigated in the trial could not be determined unambiguously from the trial publication. While post-randomisation intercurrent events that could affect interpretation of outcome data occurred in most trials (95%; eg, treatment non-adherence, use of rescue therapy or mortality), the lack of clarity in handling these was the main driver for uncertainty on the question being answered. Where the primary trial question could be unambiguously determined from the reported methods (46%), most trials considered the occurrence of such intercurrent events as irrelevant in the calculation of the treatment effect and looked at the effect of the intervention regardless (ie, they used a treatment policy strategy). Other trials alternatively looked at how the intervention under study performed in a hypothetical scenario (eg, if the intercurrent event did not occur) or used a composite approach to incorporate the occurrence of intercurrent events into the outcome. Because the answers to different questions can result in different views on treatment benefit, the trial question should be explicitly stated by including statement of the estimand to avoid misinterpretations.

Strengths and limitations

We conducted a systemic search and followed a pre-registered protocol.16 A standardised and piloted form was used for data extraction, which was conducted in duplicate by experienced trial statisticians using similar methods used in a recent review of estimands in protocols.17

We only included articles from six high impact medical journals, all of which follow the CONSORT statement, so we may have found worse reporting around estimands had we included a wider range of journals that did not endorse CONSORT. However, previous research indicates that despite journal endorsement of reporting guidelines, reporting might still be suboptimal.152627

For many trials, we were able to infer what the estimand was from the study methods. However, we had no way of knowing whether the question addressed by the methods corresponded with what the trial investigators wanted to know. Without clear specification of estimands, it is impossible to assess whether the conducted analysis was appropriate for the originally targeted question.

Research in context

Despite the growing recognition of the importance of precisely defining the research question,1228293031323334353637 this review shows that the use of estimands is still far from routine. Many of the included trials would have been designed before the ICH E9(R1) publication (draft published 2017,37 final publication 201912), and adoption by regulatory agencies worldwide (ICH E9(R1) adopted by ICH members Switzerland and Singapore in November 2019, Europe and Canada in July 2020, Taiwan in February 2021, US in May 2021, China in January 2022; and currently in the process of adoption for Korea, Japan, and Brazil38). Therefore, few trials explicitly stated the primary estimand. Of note, many of these trials were reported in line with the CONSORT guidelines, and so were reported according to best practice at the time. However, CONSORT guidelines were published before ICH E9(R1) and do not require trials to specify estimands, only the trial objective and how the numerical result (the estimate) was calculated (the statistical method or estimator). We have shown that this limited requirement does not always enable one to unambiguously infer what question was investigated. Therefore, we recommend that in any future update, estimands be explicitly incorporated into the CONSORT statement.15 More recent guidelines for the contents of statistical analysis plans in early phase trials include specification of estimands.39

However, this review found that some trials defined their estimands in their protocol, which differs from a recent review of protocols published in October 2020 in Trials and BMJ Open, where no protocols explicitly defined the estimand, and estimands could be inferred in only 26% of protocols.17 Use of estimands in this review might have been higher as we considered the six leading general medical journals and non-published protocols submitted as supplementary material.


Researchers should describe estimands in trial reports so that the precise research questions being addressed for medical interventions can be understood by all. While 46% of primary estimands could be inferred from the reported methods by our statistical reviewers, inferability is likely to be lower for typical clinical readers and other non-methodologists, including patients reading trial results. Specifying estimands has a clear benefit here: it breaks down the details behind technical language, enabling transparent interpretation for all without the need for statistical knowledge or input.

Although certain aspects of the estimand seem new and potentially difficult to specify (such as intercurrent events), in practice, the events encapsulated by this label (eg, treatment discontinuation) have been around decades and have always required thought. Rather than leaving readers to guess how these have been handled (which might turn out to be incorrect), it is useful to clarify what research question has been used with respect to such events. Trialists should carefully consider plausible intercurrent events for their individual trial setting. Examples of intercurrent events are provided in ICH E9(R1) and we have summarised those identified in this review in figure 2 and eTable 4. Certain types of trials will have other specific events to consider.

No strategies for handling intercurrent events (table 2) can be universally recommended. These will be context specific, depending on the study objectives and stakeholders and require a multidisciplinary discussion during initial trial planning to establish. ICH E9(R1) indicates how the disease under study, clinical context (eg, availability of other treatments), administration of treatment, goal of treatment (eg, symptom control or cure), and experimental situation (eg, whether it differs to that anticipated in clinical practise) should be considered when establishing the strategy.

In this review, the intercurrent event whose handling was most often not inferable was mortality—generally, the treatment policy strategy does not apply to terminal intercurrent events such as death, because patient outcomes do not exist after death.12 When participants who die are excluded from trial analysis, it is not clear whether the intention is to estimate a hypothetical treatment effect if deaths did not occur, or the treatment effect for the subset of patients who would survive only. Because the resulting estimates are likely to differ, the handling of mortality needs to be explicitly stated where relevant.

In general, when considering a hypothetical strategy, researchers should ensure that the hypothetical scenario is clinical relevant and justified. We found that some trials assessed the treatment effect in a hypothetical setting if participants continued to take treatment despite adverse events; it is debatable how clinically relevant such a scenario is. The principal stratum strategy affects who the trial result applies to (the population attribute). Thus, the generalisability of results should be given careful consideration to ensure relevance for clinical practise in the light of the strategy used. Finally, while-on-treatment and composite strategies both affect the definition of the outcome variable, so the impact on interpretability of the trial must be thought through when such strategies are used.

Although we found some evidence of estimands being specified in the protocol, it is not realistic to expect that readers of the article, including practicing clinicians and patients, will seek this out. Moreover, these documents are not always made available with results.4041 For the four trials that fully stated their primary estimand in the protocol, none of the main results articles or their supplementary appendices mentioned that the estimand could be found there. We believe the estimand, including all five attributes, should be clearly stated in the results article or in a supplementary appendix and referenced in the main article, to avoid misinterpretation. Reviewers of trial results articles should have this issue in mind to allow fully informed decisions to be made by all trial stakeholders. CONSORT guidelines should be updated to mandate reporting of estimands. These actions will ensure that there is no room for misinterpretation of results, and is in line with ICH E9(R1) recommendations that are now adopted by regulatory agencies. Use of estimands will help future reviewers evaluate whether appropriate methods have been used.


Understanding the research question being investigated in a trial is essential for informed decision making, but most often it is not clear precisely what the question is. Use of estimands can help clarify the precise study question. Trialists should explicitly describe estimands in trial reports, thereby allowing all stakeholders (including clinicians, patients and policy makers) to make fully informed decisions about medical interventions.

What is already known on this topic

  • In randomised trials, events after randomisation (such as intervention discontinuation or use of additional medications, termed intercurrent events) create ambiguity about how to define and interpret the treatment effect

  • To deal with such issues and avoid misinterpretations of results, new international trial guidelines (ICH E9(R1)) have called for a precise description of the research question the trial aims to address (ie, the estimand) to be provided

  • How often the precise trial question can be understood from the main results article and what questions are being used in trials is currently unclear

What this study adds

  • For most trials, the specific research question being investigated could not be understood from reported methods

  • Clear reporting of estimands is necessary in trial reports to avoid misinterpretation and to understand precisely what has been estimated, which is required for informed decision making around medical interventions in policy and medical practice

Ethics statements

Ethical approval

Ethical approval was not required for this systematic review of published randomised trials.

Data availability statement

The datasets used during the current study are available from the corresponding author at s.cro{at} fon reasonable request.


We thank the public contributors who were members of the HEALTHY stats public involvement group (NIHR300593), including Ania Henley (public co-chair), Paul Hellyer, Yasmin Rahman, Manos Kumar, and Joanna C; and the unnamed members of the public who attended and shared their opinions and thoughts with us at the people’s research cafe run by the NIHR Imperial Patient Experience Research Centre.


  • Contributors: SC conceived of this study. SC, BCK, JRC, IRW, and VRC developed the protocol and data extraction forms. SC, BCK, SR, and ACS extracted the data. SC analysed the data and wrote the first draft of the manuscript. All authors revised the manuscript. All authors read and approved the final manuscript. All authors had full access to all of the data (including statistical reports and tables) in the study and can take responsibility for the integrity of the data and the accuracy of the data analysis. SC is the study guarantor. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.

  • Funding: SC, advanced research fellow (reference NIHR300593) is funded by the National Institute for Health and Care Research (NIHR) for this research project. BCK, IRW, and JRC are funded by the UK Medical Research Council (grants MC_UU_00004/07 and MC_UU_00004/09). ACS is funded by the Department of Biostatistics and Health Informatics and the NIHR-MRC Trials Methodology Research Partnership. The views expressed in this publication are those of the author(s) and not necessarily those of the NIHR, NHS, or UK Department of Health and Social Care, or GlaxoSmithKline. The funder had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at and declare: SC had financial support from the NIHR for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • The lead author (the manuscript’s guarantor) affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as originally planned (and, if relevant, registered) have been explained.

  • Dissemination to participants and related patient and public communities: We intend to disseminate the importance of understanding the trial question to a public audience to facilitate the understanding of trial results, and will seek further patient and public involvement from the HEALTHY STATS public advisory panel in the development of an appropriate method of dissemination. Dissemination to the wider clinical trial community will be undertaken via presentations at relevant conferences and seminars.

  • Provenance and peer review: Not commissioned; externally peer reviewed.

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