Ratioasteikko

Ratioasteikko, or the ratio scale, is the highest level of measurement in classical measurement theory. Data on a ratio scale are numeric quantities that have a true zero point, which means the zero value signifies the absence of the quantity. The scale also features equal intervals between adjacent values, and all arithmetic operations—addition, subtraction, multiplication, and division—are meaningful. As a result, both differences and ratios between measurements can be interpreted.

Typical examples include physical quantities such as length, mass, duration, and counts, as well as age or temperature measured in Kelvin. On a ratio scale, one can say that a measurement of 10 units is twice as large as 5 units, and that the zero point corresponds to none of the quantity being measured.

Ratio data contrast with interval data, where equal intervals exist but there is no true zero (for example, temperature in Celsius or Fahrenheit). With ratio data, many statistical procedures are appropriate, including calculation of means, standard deviations, and coefficients of variation, as well as geometric means and regression models that rely on multiplicative relationships. Some transformations, such as logarithms, are meaningful only for positive ratio data.

In practice, researchers design measurements to meet ratio-scale criteria when possible, but not all variables naturally do. Understanding the scale is important for selecting suitable analytical methods and for interpreting results correctly.