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

Following up on Vince's solution, you can also do this with plyr or by:

ddply(df, .(years), function(x) transform(x, percentile=ecdf(x$scores)(x$scores)))