Nonresponse Bias — Definition, Formula & Examples
Nonresponse bias is the distortion in survey results that happens when the people who choose not to respond are systematically different from those who do respond. It means your collected data no longer accurately represents the full population you intended to study.
Nonresponse bias is a form of selection bias in which the subset of sampled individuals who fail to participate in a survey or study differs in relevant characteristics from those who do participate, causing the sample statistics to be unrepresentative of the target population parameters.
How It Works
Nonresponse bias arises during data collection, not during sample selection. Even if you draw a perfect random sample, bias enters if certain groups are less likely to reply. For example, people with strong opinions may respond at higher rates, while busy or indifferent individuals ignore the survey. The key question is always whether nonrespondents would have given different answers than respondents. If yes, your results are biased. Researchers combat this by sending follow-up reminders, offering incentives, or comparing known demographics of respondents to the full sample frame.
Example
Problem: A school mails a satisfaction survey to all 800 parents. Only 200 parents return it, and 170 of those 200 report being satisfied (85%). The school concludes that 85% of all parents are satisfied. Identify the potential bias.
Step 1: Calculate the response rate.
Step 2: Consider who is likely not responding. Parents who are dissatisfied may feel disengaged or distrustful and choose not to reply. Parents who are neutral may simply not bother. The 600 nonrespondents likely include a higher proportion of dissatisfied or indifferent parents than the 200 who replied.
Step 3: Assess the direction of bias. Because dissatisfied parents are underrepresented among respondents, the 85% satisfaction figure likely overstates the true population satisfaction rate.
Answer: The survey suffers from nonresponse bias. The 85% satisfaction rate likely overestimates the true proportion because dissatisfied parents were less likely to respond.
Why It Matters
On the AP Statistics exam, free-response questions regularly ask you to identify sources of bias in a study design. Recognizing nonresponse bias — and explaining its likely direction — is a common way to earn full credit. In professional polling and public health research, low response rates can invalidate expensive studies if nonresponse bias is not addressed.
Common Mistakes
Mistake: Confusing nonresponse bias with response bias.
Correction: Response bias occurs when people do respond but give inaccurate answers (due to wording, social pressure, etc.). Nonresponse bias occurs when people fail to respond at all, and those nonrespondents differ systematically from respondents.