Seasonal Variation: Methods of Measurement and Tests of Significance

Report Introduction: Regular seasonal variation may be removed approximately from a series by many different methods. Usually each method has some feature of merit, relating to simplicity or exactness, which makes its use preferable in certain types of analytical problems. Differences in results obtained from the use of these alternative methods are frequently small and of little importance. However, minor discrepancies between series that have been adjusted by different methods can be an annoyance and there is considerable need for some sort of standard method to use in this type of work. The following method is recommended for use in technical bulletins and reports. A test of significance is given, and procedures are outlined for determining whether the seasonal pattern has changed during time and for measuring the degree of reliability of the index of any month and of the difference between successive months. It is realized that for certain purposes an index of seasonal variation based on other methods may be desirable.