What the exam asks
On the AP Statistics exam, you must be ready to perform these tasks (Skill 2.A):
- Identify/classify the specific type of bias or nonrandom sampling method present in a study description.
- Explain how a flawed sampling or surveying method leads to a systematic error.
- Determine and justify the direction of the bias—whether the sample statistic ( or ) will likely overestimate or underestimate the true population parameter ( or ) in context.
Core ideas
What is bias?
In statistics, bias is a systematic tendency of a sampling method or survey design to produce sample statistics that are consistently larger or consistently smaller than the true population parameter.
Nonrandom sampling methods
When researchers skip random selection, the method introduces potential bias, because chance does not decide who is chosen:
- Convenience sample: Selecting individuals from the population who are easiest to reach (for example, surveying the first 50 students who walk into the library). Because these individuals usually share traits that differ from the broader population, the sample is unrepresentative.
- Voluntary response sample: Inviting an entire group to respond and letting individuals choose whether to participate (such as online polls, call-in radio shows, or customer review links on receipts). This creates voluntary response bias because people with strong, often negative opinions are much more likely to take the time to respond.