I am conducting a set of studies (S1, S2, S3) that include both quantitative (behavioral experiments) and qualitative (questionnaire). The behavioral experiments measure different variables and functions, which are correlated to the constructs in the questionnaire. The sample (participants) for the studies might change. The studies are conducted at different time points say T1, T2, T3, etc. with a time interval of around 3-4 months. Due to different time points, variables like experience, moods, emotions, etc. might vary for an old participant. In that case, do I need to readminister the questionnaire for S2 and S3 or use the one from the previous study (S1)?
A little more information would be helpful. Are you interested in examining whether and how participant scores change over multiple testings? If so, you should use a repeated-measures design where the SAME participants are measured at each time point. If you're concerned about practice effects, you can create alternate versions of the measure and have each participant complete a different version each time. Though, given that there are several months between testing times, there is little concern for practice effects. Unless there is a compelling theoretical reason to control for participant mood -for instance, if mood has a strong influence on your DV- I don't think controlling such variables is necessary. There will always be individual differences in a sample (between-subject variability), and also subtle differences across repeated testings (within-subject variability), but you have to use your judgment whether such differences need to be taken into account or not.
If you are NOT interested in examining whether/how scores change over time, I would use DIFFERENT participants for each study. By including a mix of new and old participants, you introduce confounding factors. (An exception is if you want to compare a control group (only measured once on DV) to a treatment group (measured multiple times on DV)).
Blaine Tomkins No, I am not examining how scores change over time. The experiments in two studies measure different functions (different DVs). Study 1 examines distractor interference while study 2 examines response inhibition between two different groups with no treatment in any groups. There is a possibility that the same participants are repeated in these groups due to a limited sample. Can I use the questionnaire data from the previous study for those participants?
If these are two entirely separate studies, I see no problem with including some of the same participants (and their data) in both studies.
Of course. For example: in any "test-retest" model (before a Clinical Therapeutic intervention, to see if the subject has improved -according to the indicators of said Questionnaire or Scale- or in an experimental situation in which the Experimental Group and the Group Control, one or more Scales or Questionnaires are applied, some type of intervention is carried out or certain stimuli are applied to the Experimental Group and the Control Group and, then, the same Scales, Inventories or Questionnaires).
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