I'm looking at the effect of the drug on lung function. What test should I perform for pre and post control vs pre and post treatment? Control group had measurements taken before and after recieving placebo and same for the tratment group, however they were administered a drug. Different subjects were used in control and treatment groups.
I'm very thankful for help.
Hello Sandra,
It depends on how your DV (lung function) is quantified. If you can consider it as a continuous variable of interval strength or better, then it sounds to me as if a one-way (treatment vs. control), ancova would be a logical option. Pre-measures would be used as the covariate; post-measures as the DV.
Good luck with your work.
Analysis of covariance (ANCOVA) combines regression with ANOVA, which is relatively robust against violating the assumptions of normality and homogeneity of variance. However, before running ANCOVA, as suggested by David Morse, you need to make sure that the following assumptions are met.
1. Your IV should be categorical.
2. Your DV should be continuous.
3. Your observations should be independent.
A good relevant read is the following.
Bonate, P. L. (2020). Analysis of pretest-posttest designs. Chapman and Hall/CRC. https://www.routledge.com/Analysis-of-Pretest-Posttest-Designs/Bonate/p/book/9780367578985
Good luck,
Hello again Sandra,
If it's normality (of model residuals) that you're worried about, you could always opt for bootstrap/resampling method for determining significance. Doing so allows you to test for equivalence without worrying so much about distributional assumptions.
Good luck with your work.
For pre and post you can use paired statistical tools. The size of the sample will lead you to the suitable tool like paired t-test or paired Z-test.
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