[R] Getting empirical percentiles for data

[Peter Dalgaard]

Mike Cheung <mikewlcheung at hku.hk> writes:

> I know that "ecdf" can be used to calculate the empirical cumulative
> distribution. However, I don't know how to exact the cumulative
> probabilities for each unique element. The requirement is similar to
> the "FREQUENCIES" in SPSS. Could someone help me in exacting the
> cumulative probabilities from the ecdf object? Thanks in advance!
> 
> # Generating artificial data
> x <- round( rnorm(50, mean=50, sd=10) )
> probs <- seq(0.1, 0.9, by=0.1)
> # Calculating percentiles
> x.percentiles <- quantile(x, probs)
> # Calculating percentile ranks
> x.ranks <- ecdf(x)

I don't quite see why you call it "x.ranks", but it's just a function
that you could evaluate at the step points, so continuing your code:

val <- sort(unique(x))
cbind(val,cum.prop=x.ranks(val))

I'd go for a more direct approach though:

cumsum(prop.table(table(x)))


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
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