Studying seasonality by using sine and cosine functions in regression analysis

Study objective-A statistical test that allows for adjustment of confoundin g can be helpful for the study of seasonal patterns. The aim of this articl e is to supply a detailed description of such a method. An example of its a pplication is given. Design-A statistical test is presented that retains the information on the connection of time periods by describing the seasonal pattern as one sine a nd one cosine function. Such functions can be included into a regression mo del. The resulting form of the seasonal pattern follows a cosine function w ith variable amplitude and shift. Main results-The test is shown to be applicable to test for seasonality. No t only one cosine function per time period, but also a mixture of cosine fu nctions can be used to describe the seasonal pattern. Adjustment for confou nding effects is possible. Conclusions-This method for studying seasonal patterns can be applied easil y in a regression model. Adjusted prevalences and odds ratios can be calcul ated.