aggregate() 함수는 데이터의 통계량을 계산해주는 함수이다. Splits the data into subsets, computes summary statistics for each, and returns the result in a convenient form. 함수의 기본 형태 # 'data.frame' aggregate(data, by, FUN, ...) # 'formula' aggregate(formula, data, FUN, ...) by a list of grouping elements, each as long as the variable in the data frame x. The elements are coerced to factors before use. FUN a function ..