Add a proc means step containing appropriate class and output statements, to the same SAS program used in Exercise 2.17, to create a SAS data set named stats1. The data set stats1 must contain sample means, standard errors of the means, and maximum and minimum values of the variables Birthrat, Deathrat, and Infmort calculated separately for each of the nine groups defined by combinations of levels of the category variables Birthgrp and Popgrp. Use the types statement to ensure that statistics are calculated only for combinations of levels of Birthgrp and Popgrp. Suppress printed output in proc means. Also, add a proc format step to your SAS program to define user formats for use when printing all category variables. Obtain a listing of the data set stats1. Label all new variables on output.