Correct Answer - Option 4 : Mathematical equations

__ Explanation:__

Seasonal index for time series can be calculated using the following method

1) Method of averages

2) Link Relative Method

3) Moving Average Method

But cannot be calculated using Mathematical equations.

Hence Option 4 is correct.

**Moving average method** = A moving average is a technique to get an overall idea of the trends in a data set; it is an average of any subset of numbers.

The method of moving average is useful to measure the future cost value and forcasting

**Link relative method = **This method was developed by Prof. Karl Pearson so, it is also known as karl person of seasonal variant.