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Sampling vs. Census: Understanding the Difference

Click through each step to understand the difference.

  1. What Is a Census?

    A census surveys every single member of the target population without exception β€” a national population and housing census conducted every few years is a classic example.

  2. What Is Sampling?

    Sampling surveys only a portion of the population and uses that result to estimate the whole β€” most opinion polls and TV ratings surveys use this method.

  3. Why Sampling Is More Commonly Used

    Surveying an entire population requires enormous cost and time, so if a properly sized sample can produce a sufficiently accurate estimate, sampling is far more efficient.

  4. Condition 1 for a Representative Sample: Randomness

    A sample must be selected randomly, without bias toward any particular group, for its results to reflect the characteristics of the whole population without distortion.

  5. Condition 2 for a Representative Sample: Sample Size

    If a sample is too small, random error becomes larger, so a sufficiently large sample is needed for a reliable estimate.

  6. What Is the Margin of Error?

    The margin of error shows how much a sample survey's result might differ from the true population value β€” a poll showing 40% support with a Β±3 percentage point margin of error means the true value is likely between 37% and 43%.

  7. What Is the Confidence Level?

    The confidence level is the probability that, if the same survey method were repeated many times, the result would fall within the margin of error each time β€” 95% confidence is the most commonly used standard.

Why Survey Only Part of a Population Instead of Everyone?

Many of the statistics we encounter, like opinion polls and TV ratings, are based on surveying only a portion rather than the whole population. How well a sample represents the whole strongly determines how trustworthy the survey's results are.

Sample Size Also Connects to Statistical Significance

Whether a sample was large enough and truly randomly drawn is closely tied to the separate question of whether a result should be treated as more than chance, worth keeping in mind alongside sampling itself.

Frequently Asked Questions

Is a sample survey always less accurate than a census?

Not necessarily β€” if a sample is randomly selected and large enough, it can estimate the whole population quite accurately on its own.

Should I always check the margin of error when looking at poll results?

Yes β€” if the gap between two candidates' support is smaller than the margin of error, it's hard to be confident the difference is meaningful, so checking the margin of error matters.