SLOPE 

In order to calculate beta, we need historical data for the benchmark index and historical data for the same period for the instrument for which we are trying to calculate the beta. Once the data is retrieved, we need to calculate the percentage change from one period to another.
If we are using weekly data we need to calculate the percentage change from one week to another of the benchmark index and the instrument for which we are trying to calculate the beta.

• the array of dependent variable (weekly percentage change of the instrument)
• the array of independent variable (weekly percentage change of the benchmark index)

=slope(the array of dependent variable, the array of independent variable

To calculate the the percentage change form one period to another of the benchmark and the instrument, we need to do the following calculation


Suppose we are calculating the beta on monthly basis
% change from one period to another=(Closing value month1-closing value month2)/Closing value month1


 Lets see and exhibit here (hypothetical figure)

Thanks for reading.