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    Selection Bias

    Selection bias is a systematic error arising when the people included in a survey differ from the target population in ways that affect the outcome.

    Selection bias is the umbrella category that includes nonresponse bias, self-selection bias (volunteer effect), and sampling-frame errors. It is the most common and most damaging bias in commercial research because it is invisible — the responses look fine, but they're systematically skewed. Email NPS surveys, for example, oversample customers who read your email and are engaged enough to respond, missing your at-risk segment entirely. Combat selection bias with mixed-channel recruitment, response weighting against known population parameters, and explicit reporting of who is missing.

    Example

    A product sends a Day-30 NPS email to all signups; response rate is 12%. Internal analysis shows responders' median session count was 18 vs non-responders' median of 2. Reported NPS of +35 reflects engaged users only; full-population NPS (weighted by activity) is closer to +12.

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    Related terms

    Nonresponse Bias

    Nonresponse bias occurs when the people who do not respond to a survey differ systematically from those who do, on the variable being measured.

    Response Bias

    Response bias is any systematic tendency of respondents to answer questions inaccurately, either intentionally or unconsciously.

    Convenience Sampling

    Convenience sampling recruits whoever is easiest to reach — website visitors, email-list subscribers, social-media followers — and is the dominant approach in commercial surveys despite its limitations.

    Random Sampling

    Random sampling is a probability sampling method where every member of the target population has an equal, known chance of being selected.

    Panel Survey

    A panel survey repeatedly surveys the same pre-recruited group of respondents over time, enabling within-person change analysis.

    Acquiescence Bias

    Acquiescence bias is the tendency of respondents to agree with statements regardless of their actual opinions.