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I would like to test whether there is a significant difference in outcome in a pre/post scenario.

I have the following set-up:

Measured level of concentration of A (continuous) in blood pre-death and measured concentration of A post-death for each individual (roughly 100 individuals in total).

Working hypothesis: After death, the concentration of A falls and hence should be on average lower then pre-death concentration.

So, my first impulse was to go for a paired-t-test given all assumptions are fulfilled. However, my problem arises from the fact that the post-death concentration of A is not always measured at the same time after the death has occurred. So maybe 1h after death, sometimes 12h later and so on. I wonder if this causes any problems and whether I can somehow factor in the difference in post-death time-to-test. (There are also other parameters which change on an individual basis: age, concentration, time, sex, cause of death; they are all categorical)

Is there any alternative approach to this or is the "significant difference" approach, given so many loose ends, just not reasonable at all? In the paired t-test I would just ignore all other parameters, though they might affect the concentration of A as well.

Another idea would be to use a one-way MANOVA with the dependent variables "pre-death_A" and "post-death_A" and the independent variables age, concentration, time, sex, cause of death.

I have never done this before so I am a bit hesitant. Any other suggestions?

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I would analyze the data a bit more before forming the hypothesis; run a correlation and a multiple logistic regression on time after death of sample vs. concentration (including age, etc.). If time after death matters little, make your hypothesis simple: pre vs. post. If time after death does matter, then you'll want to form your hypothesis differently. You may need to set aside a training set & re-validate using the other set to avoid "torturing the data until it confesses" so to speak.

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  • $\begingroup$ Yeah, maybe that's really a good idea to first figure out whether time is a relevant factor or can be dropped. I'll definitely look into it... $\endgroup$
    – user190080
    Oct 19 '20 at 20:39

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