# Error values for pooled groups?

We have 9 cages of mice, 5 (genetically identical, age- and sex-matched) mice per cage. They receive 3 treatments, so 3 cages per treatment. Among the ways to evaluate treatments is to monitor weight change (i.e. repeatedly weigh the individual animals). However, individual mice are not marked in the cages, so we don't know how individual mice change, other than cage averages. For example, we might have values like this ...

Cage 1, day 1: 18.1 gm, 19.1 gm, 18.7 gm, 18.5 gm, 19.5 gm
Cage 1, day 3: 19.1 gm, 18.8 gm, 18.9 gm, 18.4 gm, 20.1 gm
...

Cage 2, day 1: 18.8 gm, 19.0 gm, 18.1 gm, 18.2 gm, 19.9 gm
Cage 2, day 3: 17.3 gm, 16.9 gm, 17.1 gm, 18.2 gm, 18.4 gm
...


and so on.

Our question is of course the significance of the weight differences between the treatment groups. Is it possible to use a repeated measures approach on this? If so, how? If not, what's the most appropriate technique? What if we only have one cage per treatment instead of three?