Percentile for Each Observation w/r/t Grouping Variable

Question

I have some data that looks like the following. It is grouped by variable \"Year\" and I want to extract the percentiles of each observation of Score, with respect to t

Answer 1

You can also do something like this:

# first I'll create two dummy variables (Year, Score)
year <- rep(2001:2005, 2)
score <- round(rnorm(10, 35, 3))

# then coerce variables to data frame
d <- data.frame(year, score)

# then you can use split() function to apply
# function to each stratum of grouping variable
sapply(split(score, year), function(x) quantile(x, probs=seq(.1, .9, .1)))

Output will go something like this:

     2001 2002 2003 2004 2005
10%  34.3 32.1 34.3 29.6 36.1
20%  34.6 32.2 34.6 30.2 36.2
30%  34.9 32.3 34.9 30.8 36.3
40%  35.2 32.4 35.2 31.4 36.4
50%  35.5 32.5 35.5 32.0 36.5
60%  35.8 32.6 35.8 32.6 36.6
70%  36.1 32.7 36.1 33.2 36.7
80%  36.4 32.8 36.4 33.8 36.8
90%  36.7 32.9 36.7 34.4 36.9

You can utilize t() function to transpose rows and columns if you prefer. Writing a function will be a good way to tackle this kind of problems. I strongly recommend plyr package written by Hadley Wickam.

Hope this helps! All the best!