how to summarize by group on r how to summarize data by group in r what is data frame in r how to sum the data in r how to sum the data by group in r r how to summarize data by group how to summarize data by group in r? age in a group in r how to summarize data by group in r what's group means in r how to find the age group in r age range sd in r r how to summarize by group r how to get a group number in graph how to sum data by group how to summarize group in r how do you organize data in r how many groups in r r how to summarize the data in groups how to aggregate the data by group in r