Intervalasteikot

Intervalasteikot, known in English as interval scales, are a type of measurement scale used in statistics and the social sciences. They assign numbers to observations such that differences between values are meaningful and equal in size. The zero point on an interval scale is arbitrary and does not indicate the absence of the quantity being measured.

Key properties include: the numbers preserve order, and the gaps between adjacent values are equal, allowing meaningful addition and subtraction. However, multiplication and division are not generally meaningful for interpreting ratios on interval scales. Linear transformations of the form x' = ax + b (with a > 0) preserve the interval structure, which means rankings and differences remain consistent under such rescaling.

Examples commonly cited as interval scales are the Celsius and Fahrenheit temperature scales and many standardized test scores, such as IQ scores or scaled exam results. In these cases, equal differences correspond to equal changes in the measured attribute, but a true zero point (indicating absence of temperature or quantity) does not exist. Some quantities used in practice may be treated as interval scales, though others (like Kelvin temperatures in physics) may be considered ratio scales where a true zero is meaningful and ratios can be interpreted.

In analysis, interval scales justify the use of means, standard deviations, and other parametric methods that rely on meaningful differences. Researchers should avoid interpreting ratios of interval-scale values as having real meaning. The concept originates from measurement theory and is one of the four levels of measurement proposed by Stevens in 1946: nominal, ordinal, interval, and ratio.