Allen writes:
Having considered all the answers received so far, I have concluded that our  testing so far was  * sufficiently large for varroa

Peter Edwards sent in a very nice table showing sample size and certainty. According to his table, with a sample size of 5 bees one could be 90% certain *only* if the infestation rate was 50% or higher. I don't know how you could conclude that a sample of 5 bees from the outside of the cluster in winter was sufficient to *know* anything.
I still haven;t heard an explanation of why you would open the hives in the middle of winter, or what you were expecting to find out.

Am I missing something?

PB