Mouse sample size problem

Say I have ten cages, each with three mice inside.

Five cages have mutant mice. Five cages have normal mice. Each individual mutant cage is paired to one normal cage, because those 6 animals are littermates.

From these mice I take body weights. I then need to test whether significant body weight differences arise due to the mutation with a matched sample t-test.

The question is, does each cage count as an n of 1 or does each animal? TECHNICALLY, I know it is each cage - statistical units need to be independently ramdomised, treated, must not interfere with each other yadda yadda.

But what exactly is it I would need to demonstrate in order to justify using animals as my statistical units? Do I use an ANOVA to show that all the 5 normal cages do not significantly differ, and that all the 5 mutant cages do not significantly differ? Would this show that which cage they were in didn't matter and allow me to use individual animals as my statistical unit? Or do I use an R-squared test to show that the cage that the proportion of variation due to cage is less than some arbitrary amount?

Also, how would I perform matched sample t-test if the analysis was based on individual animals? Just randomly match mutant and normal mice from the paired cages? I guess the mean differences would still be the same regardless of exactly which mice in matched cages were compared eh?


Active Member
Hi Granty,

Yes, in the books it is writen: "independent data" ==> independent t-test, "dependent data" ==> paired t-test

I think the real prerequisite for the t-test: you should check a randomly selected sample from 2 different groups.
I think ... that if the sample you took can count as a "randomly selected" you can use the "independent t-test" in the individual level.

Of course, you can't take strong dependent data like the same mouse in the morning and in the evening as 2 samples ...

When having dependent pairs, the recommendation is to use "paired t-test" since it will reduce the standard deviation, so you will need a smaller sample. again I'm not sure that in your case it is a prerequisite.

For an extreme example, if you compare, people from 5 cities you don't have a problem to run an independent t-test despite the fact that there is some connection between the people in the same city. (yes I know the connection in the same cage may be stronger).
But even if there is a small difference between the cages, I assume it will be ok if it can be count as "randomly selected"

Does anybody have other thoughts?
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