I have data for which I can compute a sample mean. These samples are certainly not random. Is there any way to make an estimate of the population mean from a single biased sample?
The only way I can think of is to use other characteristics of the data. Say, you have collected fields X, Y and Z, and field Z happens to be the height of people. You know the distribution of the height in the population from, say Wikipedia, and then re-weight each individuals so that their weighted distribution matches of the sample is identical to the height distribution of the population. If you don't have such data, I am afraid what you want to do is impossible