Seasonality Index From Monthly History
Measure normal highs and lows by month.
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Help me calculate a seasonality index. Monthly values for at least [YEARS] years: [MONTHLY_DATA]. What the values represent: [MEASURE]. Provide steps and formulas for averaging each calendar month across years, computing an index relative to the overall average, and adjusting a recent month for seasonality. Show how to chart the pattern and how to test whether the pattern is stable across years. Explain the limits: few years of data, one-off events, and trend. Include a small invented example and a worked adjustment. Recommend a way to flag months that are outliers, and warn me not to treat the index as a forecast on its own. Add a section that applies the index to a budget, showing how to spread an annual target across months and compare actuals with a seasonally adjusted expectation. Add a short test for the index: the twelve indices should average to 1, and a method to show the adjusted series next to the raw series on one chart.
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