A little about survey bias.

Survey Sampling Bias (stattrek.com)

Bias Due to Unrepresentative Samples

A good sample is representative. This means that each sample point represents the attributes of a known number of population elements.

Bias often occurs when the survey sample does not accurately represent the population. The bias that results from an unrepresentative sample is called selection bias. Some common examples of selection bias are described below.

• Undercoverage. Undercoverage occurs when some members of the      population are inadequately represented in the sample. A classic example      of undercoverage is the Literary Digest voter survey,      which predicted that Alfred Landon would beat Franklin Roosevelt in the      1936 presidential election. The survey sample suffered from undercoverage      of low-income voters, who tended to be Democrats.

How did this happen? The survey relied on a convenience sample, drawn from telephone directories and car registration lists. In 1936, people who owned cars and telephones tended to be more affluent. Undercoverage is often a problem with convenience samples.

• Nonresponse bias. Sometimes, individuals chosen for the sample are      unwilling or unable to participate in the survey. Nonresponse bias is the      bias that results when respondents differ in meaningful ways from      nonrespondents. The Literary Digest survey illustrates      this problem. Respondents tended to be Landon supporters; and      nonrespondents, Roosevelt supporters. Since only 25% of the sampled voters      actually completed the mail-in survey, survey results overestimated voter      support for Alfred Landon.

The Literary Digest experience illustrates a common problem with mail surveys. Response rate is often low, making mail surveys vulnerable to nonresponse bias.

• Voluntary response bias. Voluntary response bias occurs when sample members      are self-selected volunteers, as in voluntary samples. An example would be      call-in radio shows that solicit audience participation in surveys on      controversial topics (abortion, affirmative action, gun control, etc.).      The resulting sample tends to overrepresent individuals who have strong      opinions.
• Random sampling is a      procedure for sampling from a population in which (a) the selection of a      sample unit is based on chance and (b) every element of the population has      a known, non-zero probability of being selected. Random sampling helps produce      representative samples by eliminating voluntary response bias and guarding      against undercoverage bias. All probability sampling methods rely on      random sampling.

Bias Due to Measurement Error

A poor measurement process can also lead to bias. In survey research, the measurement process includes the environment in which the survey is conducted, the way that questions are asked, and the state of the survey respondent.

Response bias refers to the bias that results from problems in the measurement process. Some examples of response bias are given below.

• Leading questions. The wording of the question may be loaded in some way      to unduly favor one response over another. For example, a satisfaction      survey may ask the respondent to indicate where she is satisfied,      dissatisfied, or very dissatified. By giving the respondent one response      option to express satisfaction and two response options to express      dissatisfaction, this survey question is biased toward getting a      dissatisfied response.
• Social desirability. Most people like to present themselves in a favorable      light, so they will be reluctant to admit to unsavory attitudes or illegal      activities in a survey, particularly if survey results are not confidential.      Instead, their responses may be biased toward what they believe is      socially desirable.

Sampling Error and Survey Bias

A survey produces a sample statistic, which is used to estimate a population parameter. If you repeated a survey many times, using different samples each time, you might get a different sample statistic with each replication. And each of the different sample statistics would be an estimate for the same population parameter.

If the statistic is unbiased, the average of all the statistics from all possible samples will equal the true population parameter; even though any individual statistic may differ from the population parameter. The variability among statistics from different samples is called sampling error.

• Increasing the sample size tends to reduce the sampling      error; that is, it makes the sample statistic less variable. However,      increasing sample size does not affect survey bias. A large sample size      cannot correct for the methodological problems (undercoverage, nonresponse      bias, etc.) that produce survey bias. The Literary Digest example discussed above illustrates this point.      The sample size was very large. Over 2 million surveys were completed; but      the large sample size could not overcome problems with the sample -      undercoverage and nonresponse bias.