Suppose the non-responders are identical to the responders. Then nothing is lost by trying to get them to participate again. But if they are not, replacing them

Suppose the non-responders are identical to the responders. Then nothing is lost by trying to get them to participate again. But if they are not, replacing them will lose information. > ill, busy, distracted by personal issues, absent, drunk But being ill correlates (lightly) with being older, and being drunk heavily correlates with opinions. Being absent might correlate with wealth (and not even linearly). Personal issues probably correlate with education. Add them together, and you've got a fairly large part of the population missing from your sample.