Note that this calculator calculates seasonal indices for monthly data. For quarterly data please use Seasonal Indices for Quarterly Data.
This is the continuation of a theme started in the article Analytical performance indicators.
Here we will talk about average seasonal indices - analytical indicators of time series characterizing the seasonal fluctuations
The seasonal fluctuations are annual, constantly repeating changes of the studied phenomena. During the analysis of the annual dynamics, you obtain the quantitative characteristics, reflecting the nature of the indicators' changes by months of the annual cycle.
Seasonal fluctuations are described by seasonal indices, which are calculated as a ratio of the indicator's actual value to some theoretical (predicted) level.
Where i - the number of the seasonal cycle (years), j - the season's ordinal (months).
The obtained values are subject to random deviations. That's why these values are averaged out by years, so you get the average seasonal indices for each period of the annual cycle (months)
Depending on the nature of the time series changes, you can calculate the formula with different methods.
I'll review the easiest method - the method of simple averages. The method can be used for the time series with no or negligible downward/upward tendencies. In other words, the observed value fluctuates around a certain constant value.
, average for each season j (months) for all n periods
, an average for all periods (n) and seasons (m)