Sample size and error rate validation

[Grey]

I need to have a 3rd party pass me a series of files some of which they claim may be corrupt. Those corrupt files can be recovered.

They claim a failure rate of N% (5 supposedly) but I want to validate that value by having a sample series collected. How big a sample do I need to validate the error rate of N%?

[mozchron]

You need to do power analysis. (http://en.wikipedia.org/wiki/Statistical_power)

Here is a list of online Power calculators. (http://statpages.org/#Power)