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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 the Year it is from, preferably as a vector.
Year Score
2001 89
2001 70
2001 72
2001 ...
..........
2004 87
2004 90
etc.
How can I do this? aggregate will not work, and I do not think apply will work either.
277
So how to find percentiles in R? You find a percentile in R by using the quantiles function. It produces the percentage with the value that is the percentile. This is the default version of this function, and it produces the 0th percentile, 25th percentile, 50th percentile, 75th percentile, and 100th percentile.
Percentiles indicate the percentage of scores that fall below a particular value. They tell you where a score stands relative to other scores. For example, a person with an IQ of 120 is at the 91st percentile, which indicates that their IQ is higher than 91 percent of other scores.
Step 1: Arrange all data values in the data set in ascending order. Step 2: Count the number of values in the data set where it is represented as 'n'. Step 3: calculate the value of k/100, where k = any number between zero and one hundred. Step 4: Multiply 'k' percent by 'n'.
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)))
Using ave
ave(d1$scores, d1$year, FUN=function(x) ecdf(x)(x))
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