How to determine if survey response samples are comparable? I'm looking for a method to determine if the number of survey response received from a regional office are substantial enough to compare to each other year over year.  For example, one year we might get 29 survey responses in a month from one site but only 5 responses for the same month the next year.  Can I say those are truly comparable?
I guess what I am thinking of is similar to a Gallup poll or something like that, where they test to see if their response make up approximately the same mix of people between two time frames.
The reason we are trying to do this is to avoid a survey bias, as we know some regional sites consistently perform better than others, so if our sample contains more responses from the better performing sites, our results will be skewed favorably when they are really just due to having a better response rate (more observations) from certain cites.
 A: I've totally edited this answer based on your comments, below. 
So what you have is technically called unbalanced panel data. You have a fixed set of sites that report daily productivity on a daily basis, where each site's data has missing days that are not reported. The number of missing days varies between sites, and within a site over time. Productivity is defined as the percentage of orders received and fulfilled that day.
Your suspicion is that sites will be more likely to report on days that are especially productive because they want to bump this fact up the chain. You want to calculate productivity on a monthly basis per region (collection of sites), comparing the same month across years, but you're worried that the mixture of sites that report a significant number of days in a particular month may change over time and thus not be representative of the region.
This went in a different direction that I originally thought, so I'm mainly clarifying so that someone like @Peter Flom might give better advice.
Two last questions from me: How many years of data do you have? Has the number of sites that exist, or the way they're grouped into regions changed over the time you've gathered data or has it been pretty static (except for the actual reporting, of course)?
