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A decision aid to support informed choices about bowel cancer screening among adults with low education: randomised controlled trial

BMJ 2010; 341 doi: https://doi.org/10.1136/bmj.c5370 (Published 26 October 2010) Cite this as: BMJ 2010;341:c5370

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Re:A question of validity

Paul Hewitson raises important issues around determining whether
people have made an 'informed choice' about screening.

1. Categorisation of positive/negative attitudes

Classifying attitudes as 'positive' or 'negative' is a difficult
issue and one which we carefully considered when designing the trial. We
identified 3 possible ways to deal with the attitudes construct. These
were as follows:

(1) To use the median of the scale in which positive and negative
attitudes are defined by the midpoint of the scale [1 2].

(2) To use the median split as used by the authors of the construct
[3] and the method applied in the current study. This leads to the
categorisation of people as 'positive' or 'negative' based on the median
of the overall sample;

(3) To choose a threshold (on the scale) over which we believe
attitudes are 'positive' vs 'negative'.

As Hewitson notes, method 1 may be inappropriate in the context of
screening as people's attitudes towards screening tend to be highly
positive, and few participants would fall into the 'negative' attitude
category. In the current study, we chose to use method 2, although we do
recognise that there are some limitations to this approach. We have also
applied method 3 to the trial data. This method has been applied in our
previously published decision aid work [4]. We reanalysed the trial data
using scores of 4 or above out of 5, on each item of the attitude scale
and categorised these scores as 'positive'. Score below this threshold
were categorised as 'negative'. Using this alternative approach made no
difference to our findings; 39% of decision aid participants made an
informed choice, compared to 16% in the control groups (P<0.001) a 23%
difference (very similar to our finding of a 22% difference using method
2).

A 4th approach is to determine people's values about screening using
the values clarity subscale on the decisional conflict scale. This
approach has been used by our colleagues in a mammography screening
decision aid trial [5]. Applying this approach to our trial data again did
not change our findings: the proportion of participants making an informed
choice remained approximately the same (35% decision aid groups vs 14%
control group, 21% difference). The reason we did not use the values
clarity subscale was because we felt that the attitude measure developed
by Marteau and colleagues was more appropriate in assessing people's
attitudes towards actually doing the bowel cancer screening test.

Finally, it could be that the problems associated with categorising
people as 'positive' and 'negative' about screening may lie more in how we
label attitudes. It may be more appropriate to use labels such as
'positive' vs 'less positive' or 'highly positive' vs 'moderately
positive' in the case of screening, rather than categorising people in
such as absolute terms (i.e. 'positive' and 'negative').

2. Classification of 'adequate' knowledge

Participants' overall knowledge score was based on the combined
scores for the conceptual and numeric items. Thus participants were
considered to have 'adequate knowledge' if their total score (conceptual
and numeric items combined) was 6 or more out of 12 rather than based on
proportions scoring correctly on individual items. This is a very common
practice in knowledge assessment. It was decided a priori to set the pass
mark for knowledge at 50%, however, we also examined the effects of
increasing or decreasing the knowledge pass mark on informed choice. As
described in our paper, increasing and decreasing the pass mark for
'adequate' knowledge had little effect on the difference (%) of
participants making an informed choice between the decision aid and
control groups. For example, increasing the pass mark to 75% correct (i.e.
9 or more out of 12), the difference in the percentage of participants
making an informed choice remained approximately the same (i.e. 20%)
between the decision aid and control groups.

We did not expect participants to be 'statistically or numerically
competent as the experts'. Rather, we wanted to help participants have a
better understanding of the approximate number of people who may
experience the different outcomes of screening. As described in our paper,
the marking scheme was designed to award participants who gave a ballpark
answer to the numeric questions. Thus, participants were not penalised if
they did not know the precise/exact numeric answer.

3. Reason for reduction in screening participation and participants'
perceptions of materials

Participants in the decision aid group (compared to control
participants) generally had a better understanding of their baseline risk
of bowel cancer and the absolute risk reduction (or number of lives saved)
by screening. In turn, participants' knowledge about the absolute benefit
of screening may help to explain differences in screening behaviour
between the two groups. The qualitative study (as Hewitson points out) is
indeed helping us to better understand the reduced uptake in screening
among decision aid participants. We hope to publish the findings from this
study shortly.

With regard to participants' perceptions of the information
materials, we found that 52% of the decision aid participants and 59% of
the control participants felt the information was encouraging bowel cancer
screening. None of the participants thought the decision aid or control
booklet was discouraging screening. This is perhaps not surprising given
that all participants were sent a bowel screening test kit, which some
participants may have interpreted as a sign of encouragement. In addition,
we know from our previous work that people often expect information about
screening to be in favour of screening [6 7]. Together, these factors may
have contributed to the small difference we observed in the way people
perceived the two resources.

We agree that more theoretical work is needed to examine how people
make screening decisions using high quality information, however we have
neither 'arbitrarily categorised' people nor expected lay people to have
'expert numerical understanding'.

1. Dormandy E, Hooper R, Michie S, Marteau TM. Informed choice to
undergo prenatal screening: a comparison of two hospitals conducting
testing either as part of a routine visit or requiring a separate visit. J
Med Screen 2002;9(3):109-14.

2. Dormandy E, Michie S, Hooper R, Marteau TM. Low uptake of prenatal
screening for Down syndrome in minority ethnic groups and socially
deprived groups: a reflection of women's attitudes or a failure to
facilitate informed choices? Int. J. Epidemiol. 2005;34(2):346-52.

3. Marteau T, Dormandy E, Michie S. A measure of informed choice.
Health Expectations 2001;4(2):99 - 108.

4. McCaffery KJ, Irwig L, Turner R, Chan SF, Macaskill P, Lewicka M,
et al. Psychosocial outcomes of three triage methods for the management of
borderline abnormal cervical smears: an open randomised trial. BMJ
2010;340.

5. Mathieu E, Barratt A, Davey H, McGeechan K, Howard K, Houssami N.
Informed Choice in Mammography Screening: A Randomized Trial of a Decision
Aid for 70-Year-Old Women. Archives of Internal Medicine 2007;167(19):2039
- 46.

6. Smith SK, Trevena L, Nutbeam D, Barratt A, McCaffery KJ.
Information needs and preferences of low and high literacy consumers for
decisions about colorectal cancer screening: utilizing a linguistic model.
Health Expect 2008;11(2):123-36.

7. Smith SK, Trevena L, Barratt A, Dixon A, Nutbeam D, Simpson JM, et
al. Development and preliminary evaluation of a bowel cancer screening
decision aid for adults with lower literacy. Patient Education and
Counseling 2009;75(3):358-67.

Competing interests: No competing interests

15 November 2010
Sian K Smith
Post doctoral research fellow, Screening and Test Evaluation Program
Kirsten McCaffery, Lyndal Trevena, Judy Simpson, Alexandra Barratt, Don Nutbeam
University of Sydney/University of Southampton