I have a panel dataset (denoted as fidata) including 5000 firms (with an identifier of ticker), and for each firm, there are 30 years (time variable is denoted by year) of observations of variable X. I would like to calculate the median of the X variable of these years in each firm. Which code below can help me achieve this purpose?

a.

apply(fidata$X, median)

b.

aggregate(.~year, data=fidata, median)

c.

median(fidata$X)

d.

aggregate (.~ticker, data=fidata, median)