[R] ecdf

[David Winsemius]

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