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I am setting up an experiment where I would like to determine whether there exists a difference between the participants' average self reported mental effort in some condition A and condition B.

The participants will have to do an exercise and report their invested mental effort. Because of learning effects, a participant should not perform the same exercises in both condition A and B. However, I still would like to account for the fact that participants can have a different 'base level' of invested mental effort.

To deal with this, I want to (randomly) split the sample into two groups. Group 1 will do exercise set 1 in condition A, and exercise set 2 in condition B. Vice versa, group 2 will do exercise set 2 in condition A, and exercise set 1 in condition B. This means that every participant will be in both condition A and B, but with different exercises.

However, I am unsure what test would be appropriate to compare the means in condition A and condition B. Even though each participant is in both condition A and B, it is not truly a paired design, because the exercise set is different. On the other hand, it is also not a truly independent sample design, because the same people are in condition A and B.

I am only interested in the difference between condition A and B, so not necessarily in the effect of the exercise set (1 or 2). However, should I still perform e.g. a mixed ANOVA? If so, I am a bit unsure on how to define the factors. Or does there exist another test that is appropriate for this type of experiment design?

Any help would be appreciated!

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  • $\begingroup$ You could estimate a subject effect for each subject (if you're prepared to assume no interaction between subject and condition), which should reduce the error variance if the variation between subjects is substantial; alternatively you may be better to use a mixed effects model. $\endgroup$ – Glen_b Jan 17 at 17:29
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You have a between participant factor (Group) and a within participant factor (condition A/B). Your mixed effects model will give you the effect of condition. The group effect is unlikely to correspond to any effect of scientific interest if I understand your problem. I would suggest ensuring that you have equal numbers in the two groups so that equal numbers of people in each condition do each exercise.

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  • $\begingroup$ Thanks! Just to verify: the assigned group already implies which exercise set was assigned for each of the two conditions, so I do not have to take into account exercise set as a separate factor, right? Second, does it matter whether each entry consists of [participant, group, condition, exercise, mental effort] or [participant, group, condition, average mental effort over exercises]. As I am not interested in the effect of each question, I suppose the latter is sufficient, but I'm not sure. $\endgroup$ – Hilde Jan 18 at 9:34

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