Rapid responses are electronic comments to the editor. They enable our users
to debate issues raised in articles published on bmj.com. A rapid response
is first posted online. If you need the URL (web address) of an individual
response, simply click on the response headline and copy the URL from the
browser window. A proportion of responses will, after editing, be published
online and in the print journal as letters, which are indexed in PubMed.
Rapid responses are not indexed in PubMed and they are not journal articles.
The BMJ reserves the right to remove responses which are being
wilfully misrepresented as published articles or when it is brought to our
attention that a response spreads misinformation.
From March 2022, the word limit for rapid responses will be 600 words not
including references and author details. We will no longer post responses
that exceed this limit.
The word limit for letters selected from posted responses remains 300 words.
I do not agree with Dr. Satyanarayana Labani that convenience sample should not be done in observational studies. Sometimes there is no option left. For example, we have to do a cohort study for teenage pregnancy (study cohort) and compare them with the pregnancy outcome with the control cohort of women 20-25 years of age group (the safest for pregnancy) then the feasible study would be hospital based and the sample has to be convenient sample.
P Sedgwick' s articles with questions and answers along with explanations are interesting. In answering the present article's question #2 whether the sample was prone to selection bias, the author referred to his previous article on convenience sampling (BMJ 2013;347:f6304). In that article the author advised or advocated the use of convenience sampling in conjunction with most study designs and not solely with clinical trials. As he stated - despite the potential limitations, convenience samples are used to recruit patients by many investigators. It is surprising to note such advice to use this type of sampling from a statistician for any study design and not solely clinical trials. If it is for clinical trials it may be acceptable but observational studies use of convenience sampling destroys external validity and restricts use of statistical methods of inference. Please would the author clarify this. We think that convenience sampling is not recommended for study designs other than clinical trials.
Competing interests:
No competing interests
14 August 2015
Satyanarayana Labani
Scientist
Smita Asthana
institute of cytology and preventive oncology (icmr) noida (uttar pradesh), india
Re: Bias in randomised controlled trials: comparison of crossover group and parallel group designs
I do not agree with Dr. Satyanarayana Labani that convenience sample should not be done in observational studies. Sometimes there is no option left. For example, we have to do a cohort study for teenage pregnancy (study cohort) and compare them with the pregnancy outcome with the control cohort of women 20-25 years of age group (the safest for pregnancy) then the feasible study would be hospital based and the sample has to be convenient sample.
Competing interests: No competing interests