These presentations introduce R functions that can be used by epidemiologists to summarize data by rows and columns, groups observations to conduct analyses and cross tabulate counts and proportions of categorical data. The slides for this material can be found here.

**Marginal Analysis with R Part 1**

Using the base R apply() function on matrices and arrays to conduct marginal summary analyses like calculating row and column statistics.

**Marginal Analysis with R Part 2**

Using lapply(), sapply() and mapply() to conduct marginal summary analyses on more complex R objects like lists.

**Stratified Analysis in R**

Using the base R functions tapply(), by() and aggregate() to group observations and conduct stratified analyses.

**Frequencies and Crosstabulations in R**

Using the base R function table() to count frequencies and cross tabulate counts and proportions of categorical variables. Introduction to the CrossTable() function in the "gmodels" to return frequency and cross tabulations formatted like SAS PROC FREQ.