A number of different methods are available to estimate the trend; however, the suitability of these methods largely depends on the nature of the data and the purpose of the analysis. To measure a trend which can be represented as a straight line or some type of smooth curve, the following are the commonly employed methods:
(1) Freehand smooth curves
(2) Semi-average method
(3) Moving average method
(4) Mathematical curve fitting
Generally speaking, when the time series is available for a short span of time in which seasonal variation might be important, the freehand and semi-average methods are employed. If the available series is spread over a long time span and has annual data where long term cycles might be important, the moving average method and the mathematical curve fitting are generally employed.