What Do U Mean By A Seasonal Index?

A measure of seasonal variation which is usually computed in index form, is called seasonal index. To compute seasonal indicies, the components of time series are assumed to follow the multiplicative model. Simple averages of the monthly or quarterly values over a period of years are known as seasonal averages. When these seasonal averages are expressed as percentages of the average of all seasonal averages, they are called seasonal indicies. In symbols;

Seasonal index = seasonal average / grand average * 100

Explain the ratio to link relative method of measuring seasonal variation