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Calculating cumulative probability using cgHistoPlot [message #78261] Tue, 08 November 2011 10:10 Go to previous message
Xin Tao is currently offline  Xin Tao
Messages: 40
Registered: April 2011
Member
Hi all,

I'm using cghistoplot, and I'm confused by the cumulative probability
calculated by cghistoplot. I'm wondering whether any one can give me
some help here.

Suppose we have histdata like this: [2, 0 , 0, 3, 5, 4], then if
we use the way of cghistoplot to calculate the cumulative probability
like this:

cumTotal = Total(histData, /CUMULATIVE) ;; gives us
[2.00000, 2.00000, 2.00000, 5.00000, 10.0000,
14.0000]
probability = Scale_Vector(cumTotal, 0, 1) ;; gives us
probability = [0.00000 0.00000 0.00000 0.250000
0.666667 1.00000]

This is kind of counter-intuitive to me, because the first value of
histdata is clearly 2, but the probability is 0 until the 4th value.
However, I'm not experienced in data anlaysis, and I might have
misunderstood something about "cumulative probability" here.

It seems to be more natural to me to define the cumulative probability
in the following way:

probability = total(double(hist)/total(double(hist)), /cumula)

Am I right?

Xin Tao

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