CODEWITHSUNDEEP
rgamma-distribution
sam_rox
sam_rox

Reputation: 747

how to use qgamma in R

The gamma distribution with a shape parameter k and a scale parameter theta is defined by enter image description here= enter image description here

In R If I want to find the quantile at 0.05 probability for a gamma distribution with Gamma(10,0.5)

I used 

> qgamma(0.05,shape=10,scale=0.5)
[1] 2.712703

but this is not the value I want. The desired value I get when I use, 

qgamma(0.05,10,0.5)
[1] 10.85081

So what is the difference of qgamma(0.05,10,0.5) and qgamma(0.05,shape=10,scale=0.5).
Why do I get two completely different results?

Upvotes: 3

Views: 14202

Answers (2)

Sorif Hossain
Sorif Hossain

Reputation: 1351

     qgamma(x,shape,rate,scale=1/rate)
    #here when we use it as a process
    qgamma(0.05,10,0.5)
    #it takes 0.5 as a rate but not as a scale
    qgamma(0.05,shape=10,scale=1/0.5)
    #here 1/0.5 is a scale value but  not rate
    qgamma(0.05,10,shape=1/0.5)
    10.85081
    qgamma(0.05,10,0.5)
    10.85081

Upvotes: 5

IRTFM
IRTFM

Reputation: 263411

Read the help page: Scales is the fourth parameter of qgamma. The third parameter is rate = 1/shape. If you want to call qgamma with positional matching of parameters then it should be:

> qgamma(0.05, 10, 1/0.5)
[1] 2.712703

Upvotes: 5

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