Sampling and Measurement Bias

Question 1

What is selection bias and when does this occur?

Answer 1

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.

Question 2

When is under-coverage a particular problem?

Answer 2

Under coverage occurs when some members of the population are inadequately represented in the sample. It is often a problem with convenience samples, such as with what happened with the Literary Digest (i.e., didn’t reach low-income voters who tended to be Democrats)

Question 3

What are non-response bias

Answer 3

Non-response bias is the bias that results when respondents differ in meaningful ways from non-respondents (e.g., mail surveys - response rate is often low, making them vulnerable to this bias)

Question 4

What is sampling error?

Answer 4

The variability among statistics from different samples is called sampling error.

(I’ve also seen it written as, in essence, the deviation of the selected sample from the true characteristics.

Question 5

Under what circumstances might sampling error increase?

Answer 5

• Biased sampling procedure
• Chance through randomization/ probability sampling (used to minimize sampling process error, but it is still possible that all the randomized subjects are not rep. of the pop.)

Question 6

What impact does sample size have on sampling error?

Answer 6

Increasing the sample size tends to reduce the sampling error; that is, it makes the sample statistic less variable.

Question 7

What impact does increasing sample size have on survey bias?

Answer 7

Increasing sample size does not affect survey bias.

Question 8

Voluntary response bias

Answer 8

This occurs when sample members are self-selected volunteers, as in voluntary samples (e.g., call-in radio shows that solicit audience participation in surveys on controversial topics…over reps indvs w/ strong opinions)