Interval data identify subjects by non-relative rank in a scale. Numbers are assigned to data according to a scale where intervals are constant throughout the entire scale. Some examples may include IQ, Achievement, or Aptitude. Most educational measurement is done using Interval class data. Interval data may be properly described with analyses of statistical Mean, Median, Mode, Standard Deviation and Variance.
Ratio data identify subjects by quantification.
Numbers are assigned so that statements of ratios can be made. Numbers here represent equal units, and the range from an Absolute Zero point. Because of this, we can establish exact relationships between elements measured on such a scale. Some examples may include Time, Weight, Distance, or Temperature. The statistical analyses and descriptions are the same as for the Interval class of data.