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Definition of seasonal index numbers
In time series analysis, seasonal indices are index numbers that represent the typical seasonal fluctuations within a year in the form of the relative deviation from the annual mean. They can be represented either as an index number related to the annual average as 100 or as a percentage of the annual total that is equated with 100.
rigid seasonal indices
Depending on the method of calculation, a distinction can be made between fixed and variable seasonal indices. “Rigid indices are to be regarded as those that are calculated as a function of time, i.e. for which a complex of causes is assumed to be effective in time.
variable seasonal indices
Variable indices, on the other hand, are those in which the complex of causes is broken down into the individual factors and the relationship between the respective factor and the seasonal result is then checked using regression technology.