A number of different methods are available to estimate the trend; however, 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, and
(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, having annual data, where long term cyclic might be important, the moving average method and the mathematical curve fitting are generally employed.