I obtained the historical CPI data available in monthly and quarterly format from a data repository. It seems that the Q1, Q2, etc. data are the average of the monthly data.
I thought that if I were to get quarterly data from monthly data, I can just take the March, June, Sept, and December closing figures. If I were to get the quarterly data for stock price or stock index, should I can just take the 4 month closing price or would I have to take the average of 3 months in each quarter?