Models with Seasonality but No Trend

When the time series average does not change over time (stationary), but is subject to seasonal fluctuations, the appropriate model has seasonal parameters but no trend.

Seasonal fluctuations are assumed to balance out over periods of length m, where m is the number of seasons, For example, m=4 might be used when the input data are aggregated quarterly. For models with additive errors, the seasonal parameters must sum to zero. For models with multiplicative errors, the product of seasonal parameters must be one.