iPod Track Listening & Biological Samples— Within-Subjects Test/Regression?

I am analyzing data in which participants listened to iPods and provided biological samples on the same days. The iPod Listening Intervention was 1 week. The biological sample collection was 4 days within the 1 week. There was no intervention/control group. All participants in study were given iPods and materials for biological sample collection.

Research Question: Does listening to the iPod affect biological samples on days listened?

I’m looking to match dates of collection with listening dates to see if there is a difference between the biological samples (continuous variable) on days listened and days not listened. Importantly, some participants did not listen at all, so this may provide useful data. Some participants listened every day of the intervention while others only listened a few days.

I am assuming that participants who listened to tracks on the days of collection will provide healthier bio samples whereas days not listening will not be as healthy.

I’m very interested in this research question and was given an awesome dataset to work with, but don’t know exactly where to start. It seems like a within-subjects design but potentially could be measured by regression.

(I have already run regressions examining total listening time with the biological sample, but am looking to answer a slightly different question).

Oh dear. Your design does not seem to be appropriate for this. If you want to definitively test the effect of ipod listening, a control group should definitely be used.

However, you could still analyze the relationship between total ipod listening time (independent variable) with the biological outcomes, with HLM. Do note that the individuals are the nesting variable.