Problems with Statspack

Everyone's talking about it - statspack isn't perfect. Driven from Jonathan Lewis to Doug Burns to Daniel Fink's OptimalDBA I thought it was worth pointing out that as a snapshot technique, your knowledge of the value of the increment is uncertain.

Suppose snapshot N of metric has value s(N)
lets call the increment N to N+1 x(N)

All we can say for sure is:
A) if f(N+1) <>= f(N+1)
B) if f(N+1) >= f(N) then x(N) >= f(N+1) - f(N)

In normal use (when the shared pool is stable), we only see the case B, and we replace the >= with =. But if your shared pool is being trashed (see earlier posts) then remember uncertainty, and consider building case (A) into your reports.