Here’s how we can quite easily and flexibly create tables of descriptive statistics in R. Of course, we can simply use summary(variable_name), but this is not what you’d include in a manuscript — so not what you want when compiling a document in knitr/Rmarkdown.
First, we identify the variables we want to summarize. Often our database includes many more variables:
vars <- c("variable_1", "variable_2", "variable_3")
Note that these are the variable names in quotes. Second, we use lapply() to calculate whatever summary statistic we want. This is where flexibility kicks in: have you ever tried to include an interpolated median in such a table, just as easy as the mean in R. Here’s an example with the mean, minimum, maximum, and median:
v_mean <- lapply(dataset[vars], mean, na.rm=TRUE)
v_min <- lapply(dataset[vars], min, na.rm=TRUE)
v_max <- lapply(dataset[vars], max, na.rm=TRUE)
v_med <- lapply(dataset[vars], median, na.rm=TRUE)
Too many digits? We can use round() to get rid of them. There’s actually an argument ‘digits’ in the kable() command we’ll use in a minute that in principle allows rounding at the very end, but unfortunately it often fails on me. Rounding:
v_mean <- round(as.numeric(v_mean), 2)
Now we only need to bring the different summary statistics together:
v_tab <- cbind(mean=v_mean, min=v_min, max=v_max, median=v_med)
And add useful variable labels:
rownames(v_tab) <- c("Variable 1", "A description of variable 2", "Variable 3")
and we use kable() to generate a decent table:
kable(v_tab)
If this looks complicated, bear in mind that with no additional work you can change the order of the variables and include any summary statistics. That’s table A1 in the appendix sorted.
Didier Ruedin 