The file P02_18.xlsx contains daily values of the S&P Index from 1970 to 2009. It also contains percentage changes in the index from each day to the next. a. Create a new variable President that lists the U.S. presidents Nixon through Obama on each date. You can look up the presidents and dates online. b. Use StatTools to find the mean, median, standard deviation, and first and third quartiles of % Change, broken down by President. Also, create side-byside box plots of % Change, again broken down by President. Comment on the results.