The seasonal mean for position s is calculated as:
The multiplicative seasonal index compares each seasonal mean with the overall mean:
The additive seasonal effect measures the difference from the overall mean:
Seasonal strength is estimated from residual and total variation:
- Paste time-series observations or upload a CSV file.
- Set the seasonal period, such as twelve months.
- Choose missing-value, transformation, and outlier options.
- Customize plot labels, markers, references, and exports.
- Submit the form, then review charts and tables.
| Data frequency | Typical seasonality | Period |
|---|---|---|
| Hourly | Daily | 24 |
| Daily | Weekly | 7 |
| Monthly | Yearly | 12 |
| Quarterly | Yearly | 4 |
| Weekly | Yearly | 52 |
What is a seasonal subseries plot?
It groups observations by seasonal position. Each group reveals recurring changes across cycles.
How do I select the seasonal period?
Match the period to one complete recurring cycle. Monthly yearly data usually uses twelve.
What does the seasonal mean show?
It summarizes the average level for each season. Differences reveal systematic recurring effects.
When should I detrend the data?
Detrend when long-term growth masks seasonal differences. Preserve the original series for comparison.
What is additive seasonality?
Additive seasonality has roughly constant absolute changes. Seasonal effects are measured as differences.
What is multiplicative seasonality?
Multiplicative effects scale with the series level. Seasonal indices are expressed as ratios.
How are missing values handled?
Choose interpolation, directional filling, seasonal means, overall means, zeros, or row removal.
How are outliers identified?
Use z-score or IQR rules. Adjust the threshold for sensitivity.
What does seasonal strength mean?
Values near zero indicate weak seasonality. Values near one indicate strong recurring variation.