Seasonality

Seasonality is a regular, calendar-linked pattern that repeats within every year of a data series, such as retail sales rising every December, measured with a seasonal index.

ByHoang TruongUpdated July 11, 2026

See it move

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Seasonality is the calendar-linked pattern that repeats every year. A retailer's quarterly sales run €80,000, €95,000, €100,000 and €125,000, averaging €100,000 overall. Q4's seasonal index is (€125,000 ÷ €100,000) × 100 = 125, meaning Q4 typically runs 25% above the yearly average quarter. Deseasonalizing divides the index back out so a genuine business shift is not confused with the predictable calendar effect.

Where it fits

TopicDescriptive StatisticsCoreSubjectData Analysis & StatisticsCoreTopicMultiple Regression & InterpretationCore

The formula

LaTeX

SI = \frac{\bar{X}_{\text{period}}}{\bar{X}_{\text{overall}}} \times 100

Variables

$S I$: Seasonal index
$\overset{ˉ}{X}_{period}$: Average value for the period (e.g. one quarter) (€)
$\overset{ˉ}{X}_{overall}$: Overall average across all periods (€)

Expresses one period's typical value as a percentage of the overall average, revealing how far above or below normal that period runs.

Check yourself

PracticeCORE

A firm's quarterly sales were Q1 = €75,000, Q2 = €95,000, Q3 = €100,000 and Q4 = €130,000. What is the seasonal index for Q4?

Select an answer to check your understanding.