ecdf
On Oct 16, 2011, at 11:31 AM, gj wrote:
Hi,
Newbie here. I read the R for Beginners but i still don't get this.
I have the following data (this is just an example) in a CSV file:
courseid numstudents
101 209
141 13
246 140
263 8
321 10
361 10
364 28
365 25
366 23
367 34
I load my data using:
fs<-read.csv(file="C:\\num_students_inallmodules.csv",header=T,
sep=',')
I want to get the ecdf. So, I looked at the ?ecdf which says
usage:ecdf(x)
So I expected ecdf(fs$numstudents) to work
Instead it just returned:
Call: ecdf(fs$numstudents)
x[1:210] = 1, 2, 3, ..., 3717, 4538
After Googling, got this to work:
ecdf(fs$numstudents)(unique(fs$numstudents))
But I don't understand why if the ?ecdf says usage is ecdf(x) ... I
need to use ecdf(fs$numstudents)(unique(fs$numstudents)) to get this
to work?
Can somebody explain this to me?
ecdf() returns a function rather than a vector. You need to supply arguments to that function to get something that you recognize. Had you passed that function off to plot you would have seen that the information needed to calculate the plot is obviously "in there". If you go to the stepfun page you find that the knots function can recover some of htat information for display. > plot( ecdf(fs$numstudents) ) > knots( ecdf(fs$numstudents) ) [1] 8 10 13 23 25 28 34 140 209 If you count the knots you can deduce the quantile values (the "y- values") at which those "x-values" will start the step "dot-line"
David Winsemius, MD West Hartford, CT