Statistical Process Control (SPC) Resource Center

Measures of Variation

A population is all the data that exist for a process. A sample is a small group of data randomly taken from a population that is representative of the whole population.

Samples of data come from the population of dataThree statistical measures of variation are:


  • The arithmetic average of the sample.
  • This measures the location of the process output.

Calculating the Population and Sample Mean


  • The highest data point minus the lowest data point.
  • This is one of two measures of the variation in the process.

Standard Deviation

  • A statistic that measures how data cluster around the mean.
  • This is another measure of variation.
  • The smaller the standard deviation, the less variation there is in a process. The larger the standard deviation, the more variation there is in a process.

Calculating the Standard Deviation for a Sample and a Population