Biased or Unbiased How to survey data without adding bias.
What is bias? • In survey sampling, bias refers to the tendency of a sample statistic to systematically over- or under-estimate a population parameter.
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 sample does not accurately represent the population.
Bias – 1. Undercoverage • Undercoverage occurs when some members of the population are inadequately represented in the sample. • For example: the Literary Digest voter survey of 1936 predicted that Alfred Landon would beat Franklin Roosevelt in the presidential election. • The survey sample suffered from undercoverage of low-income voters who tended to be democrats. • The reason this happened is that survey only included people who owned cars and telephones. In 1936 the only people who owned cars and telephones were more affluent.
2. 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. • In the Literary Digest survey Alfred Landon supporters and nonrespondents, Franklin Roosevelt supporters. Since only 25% of the sampled voters actually completed the mail-in survey, survey results overestimated voter support for Landon. • Mail-in surveys are vulnerable to nonresponse data.
3. Voluntary Response Bias • Voluntary response bias occurs when sample members are self-selected volunteers. • For example, call-in radio shows that solicit audience participation in controversial topics. • The resulting sample tends to over represent individuals who have strong opinions.
Random Sampling (GOOD) • Random sampling is a procedure for sampling from a population. • The selection of a sample unit is based on chance. • Every element of the population is known • It eliminates voluntary response bias, and undercoverage bias.
Biased or Unbiased • People attending a football game were asked what their favorite sport was. • Orchestra students were asked if more money should be spent on the athletics programs. • All 1st period students were asked where they want to go to college. • People standing in line to see the latest Twilight series movie were asked what their favorite type of movie was. Biased Biased Unbiased Biased