Seasonal Index -

1. What Is a Seasonal Index? A seasonal index (also called a seasonal component or seasonal factor) is a numerical value that quantifies how a particular time period (e.g., a month, quarter, or week) compares to the average period in a seasonal cycle. It is used to measure and remove seasonal variation – predictable, recurring fluctuations that happen within a fixed period (usually one year).

[ \textAdjusted Index_i = \frac\textRaw Index_i\textMean of Raw Indices ] seasonal index

After dividing actual by the seasonal index, you get , which reveals the true trend and cycle. seasonal index

= Actual ÷ Seasonal Index.

[ \textActual = \textTrend \times \textSeasonal \times \textIrregular ] seasonal index

(We’ll skip full arithmetic for brevity – but you’d smooth the data.)