Merit of t-test vs repeated measures

In a cohort study, patients have been enrolled for a new method of treatment of a condition. The measurement of interest, for example, tumour size is taken at baseline (BL) ie before treatment, the treatment is then given and the tumour size is then measured every 4 weeks for 40 weeks (ie we have a total of 11 measurements with one BL and the rest post treatment). The analysis of interest is change in tumour size from BL to week 40.

This was recommended by someone else to be done using a paired sample t-test between BL and week 40 and supplement with a p-value and confidence interval. However I feel that using a repeated measures mixed effects model with tumour size as dependant variable and time and possibly the BL value as independent variables (ancova), would be more suitable. This way we utilise all of the data rather than just the two time points.

Please can someone explain to me the benefit of using one method over the other? Can the t-test give a realistic view of the treatment impact whilst ignoring all the middle values?

• I heard a similar question come up in a paper review. There the answer was to use ANOVA instead of a paired t-test. I'm interested to read more answers and comments, and found this interesting: theanalysisfactor.com/… Nov 1, 2018 at 23:00
• For a start, if you're going to use a t test, it will have to be a paired t test and not a two-sample t test. Nov 2, 2018 at 2:50

This is not so much a question about what test to use, but what question you're asking.

If this were a randomised controlled trial, you would be forced to define a primary outcome. For example, it might be survival at 40 weeks in those receiving drug A versus those receiving drug B. Given that you have defined the outcome, it is easy enough to choose the test statistic.

In your case, I think you and four friend are both interpreting the situation in different, yet equally valid, ways. That is, you are interested in one research question and he is interested in another.

I suggest that you think carefully about the research question you wish to answer. Then, the analysis plan should flow naturally from that point.

Good luck.