5 Bias A survey is biased if its outcome has been influenced by factors other than the one being studied. But more often bias comes from poor survey design. A typical problem is that of comparing two groups of people that are not really alike. For example, if there are more men than women in one group, and more women than men in another, the responses of the groups to any question will be influenced by the differences between men and women. In many cases these gender differences overwhelm the real subject of the study.
Similar problems apply when groups have different age profiles.
The textbook solution to the problem of bias is that of randomization. This means picking survey subjects from the population group at random. Bear in mind that if you send out questionnaires and you use all the replies, this is not a random sample of anything. This is because people who take the trouble to respond to the questionnaire are probably not respresentative of the group you sent them to. However, I would still try to select the individuals themselves at random.