how to use qgamma in R

r gamma-distribution
sam_rox picture sam_rox · Nov 29, 2015 · Viewed 8.7k times · Source

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?

Answer

IRTFM picture IRTFM · Nov 29, 2015

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

Related questions

How would you fit a gamma distribution to a data in R?

Suppose I have the variable x that was generated using the following approach: x <- rgamma(100,2,11) + rnorm(100,0,.01) #gamma distr + some gaussian noise head(x,20) [1] 0.35135058 0.12784251 0.23770365 0.13095612 0.18796901 0.18251968 [7] 0.20506117 0.25298286 0.11888596 0.07953969 0.09763770 0.28698417 [13] 0.07647302 0.17489578 0.02594517 0.14016041 0.04102864 0.13677059 [19] 0.18963015 0.23626828 How could I fit a gamma distribution to it?

Read More
Sampling from an inverse gamma distribution in R

In order to sample from an inverse gamma distribution in R, is the following the correct way to do it: #I want to sample an inverse-gamma(a,b) a = 4 b = 9 x = 1/rgamma(1,a,b)

Read More
Errors while trying to fit gamma distribution with R fitdistr{MASS}

I have a problem with fitdistr{MASS} function in R. I have this vector: a <- c(26,73,84,115,123,132,159,207,240,241,254,268,272,282,300,302,329,346,359,367,375,378, 384,452,475,495,503,531,543,563,594,609,671,687,691,716,757,821,829,885,893,968,1053,1081,1083,1150,1205,1262,1270,1351,1385,1498,1546,1565,1635,1671,1706,1820,1829,1855,1873,1914,2030,2066,2240,2413,2421,2521,2586,2727,2797,2850,2989,3110,3166,3383,3443,3512,3515,3531,4068,4527,5006,5065,5481,6046,7003,7245,7477,8738,9197,16370,17605,25318,58524) and I want to fit a gamma distribution to the data with a command: fitted.gamma <- fitdistr(a, "gamma") but …

Read More