The occurrseasonal variations, which are mainly caused by the change in seasons, are short term movementsing in a periodic manner. These fluctuations are repeated with more or less the same intensity within a specific period of one year or shorter. An index number which measures the seasonal variations, is called seasonal index number. The simple averages of the monthly or quarterly values over a period of years are known as the seasonal averages. When these seasonal averages are expressed as percentage or grand average, they are called seasonal indices.