Re: benefits of longitudinal study

On Apr 7, 1:22 pm, W <wzhang...@xxxxxxxxx> wrote:
Hello all:

I'm currently involved in a design of an experiment. We have two
options. One option is to assign a couple of animals in each treatment
group and measure them longitudinally for 20 weeks. This is typical
lingitudinal study with the same animal measured multiple times. The
other option is to do the same experiment except the animals at
different time point are different. The reason was for some variables
would be too much stree to the animals if we masure the same animal
repeatedly over time. I know statistically they are different from the
analysis side. But I don't know what pros and cons comparing the two
designs. I would appreciate if anyone could shed some light ...

Thanks,

W

For comparing treatment group means, either at a single time point or
averaged over time points, there is no special advantage to repeated
measures designs other than the reduced number of animals they use.

But if you want to look at mean *changes* between two time points for
a single group, or compare group mean changes over time, then repeated
measures designs usually have a smaller error term for a given total
number of animals (because each animal serves as its own control and
the within-group subject main effects at different times cancel) and
give a correspondingly more powerful test.

Repeated measures designs usually assume negligible carryover from one
measurement to the next. If you are planning a series of studies, then
you may want to do a method-check study where you compare never-tested
animals to previously-tested animals.
.

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