intra- and inter-subject variability

[la volta statistics]

Hi All
I got the result of a single dose, two-way crossover design comparing two
treatments under two different diet conditions. I have the AUC for each
subject under for the two treatments and the two diet condition(16
subjects).
I now was asked to calculate the inter-subject variability and the
intra-subject variability for the AUC.
Am I correct when I calculate the ratio of the AUC for the first and the
second administration of the each treatment (and each diet) as the
intra-subject variability.

I am not completely confident about the inter-subject variability. How can
that be achieved?

Any help will be appreciated.
Christian