Trend is an important component of a time series. It is the measure of permanent effect which prevails on the time series for a long period of time. Trend may be either increasing or decreasing or stable in terms of time. It represents the mathematical form and direction of variation taking place in the variable quantity yt of a given time series. It is denoted by the symbol yt’. Trend is the component obtained by eliminating the effect of short-term fluctuations (namely seasonal and cyclical) from the total fluctuations in the variable quantity of time series. In the time series of human population of any country the effect of trend is likely to be more pronounced than that of other components.

From the study of trend of a time series we can predicate a value of yt of the time series for some future value of t or we can determine the missing value for some past value of t. Also two or more sets of time series data can be compared with the help of their trends. Important decisions concerning economic policy can be taken on the basis of the trend of time series of economic data.

The estimate of the trend of time series can be obtained by the following two methods :

• Graphical method and
• Least squares method of fitting linear trend.