That's a conditional statement with which I agree--given the condition. Our experience is that response rates to voluntary surveys are hugely different from compulsory surveys, on the order of 60% to 70% for most of our surveys. Secondly, those response rates are falling.
So I would be very concerned about any comparability between data that are collected in one year on a compulsory basis--a compulsory basis with response rates close to 100%--and in another year with response rates that may be only 60% to 70%.