Calculate the grand mean of all data points across all periods.
Seasonal indices help you uncover repeating patterns in your data — whether it’s sales, web traffic, or demand. Here’s how to calculate them step by step: how to find seasonal index
Collect at least 3–5 years of data for the same period (e.g., monthly or quarterly). Calculate the grand mean of all data points
Find the average value for each season (e.g., average of all Januaries). Find the average value for each season (e
Want to go deeper? Use Excel, Python ( statsmodels ), or R for advanced decomposition (additive or multiplicative).
Here’s a clear, professional post you can use for LinkedIn, a blog, or a social media update:
📌 Example: If January’s average sales are 120 and the overall monthly average is 100, the seasonal index = 1.2 (20% above average).