系统抽样法

Systematic sampling is a simple method where samples are selected based on position at regular intervals from an ordered population list, not on characteristics like color or gender. The sampling interval is calculated by dividing population size by desired sample size. After choosing a random starting point, every nth person is selected until the required sample size is reached.

The sampling interval is determined by dividing the total population size by the desired sample size. For example, with 20,000 residence listings and a need for 400 names, the interval is 50. If the result is a decimal, round it to the nearest integer. This fixed interval ensures consistent spacing between selected samples.

Researchers must know the exact population size and the population must be randomly distributed to avoid bias. If individuals with similar characteristics cluster together—such as all tall students standing together—systematic sampling produces biased results that don't accurately represent the population.

A random starting point ensures the sample begins without bias and represents the population fairly. After selecting this first member randomly, the sampling interval is applied consistently to choose remaining samples. This randomization at the start prevents systematic patterns that could skew results.

Systematic sampling uses a fixed interval to select every nth person from an ordered list, while random sampling method selects individuals completely at random without a predetermined pattern. Systematic sampling is more convenient to execute and ensures even distribution across the population when it is randomly organized.

In a phone survey with 20,000 residence listings, researchers need 400 names. The population is numbered 1–20,000, and a random starting point selects the first name. Then every fiftieth name is chosen until 400 names are collected, creating a systematic sample across the entire phone book.

Systematic sampling is frequently used because it is convenient and straightforward to execute compared to other methods. It ensures samples are drawn without bias based on position rather than characteristics, and it provides a practical way to represent populations when measuring everyone is impractical.