I know when you do a census, you cover the entire population but when you take a sample, you select a group from the population. There can be only one census but there can be many different samples from the same population. Hence, the latter results in sampling error (because it is just one aspect of the population).
This may be a bit simplistic but I think it captures the essential differences between a census and a sample.
A consultant that my company has engaged is blurring this distinction between a census and a sample. He has essentially collected a sample, weighted it and then interpreted the findings as if they relate to the entire population. He is mostly using descriptive statistics. He has even calculated the sampling error.
My question is: Is it ever valid to treat a sample as a census? If so, what are the preconditions?