Hi R-users,
I'm fitting gamma parameters (shape and rate ) by two methods (by direct 
and and fitdistr of MASS library). I have realized the following code 
which gives me two results. However, I find that those two outcomes are 
significantly different. Is there any explanation to this case? Many 
thanks in advance for your help

> param <- function(y)
+ 
+ {
+ library(MASS)
+ 
+ b <- subset(tab[!is.na(tab$DVTDR) & (tab$DVTDR > 0),], IDAV<= y )
+ 
+ z <- b$DVTDR
+ 
+ m <- mean(z)
+ 
+ v <- var(z)
+ 
+ para1 <- list(shape = m^2/v, rate = m/v)
+ 
+ para2 <- fitdistr(z, "gamma")
+ 
+ return(list(para1,para2)) 
+ 
+ }
> 
> param("321")
[[1]]
[[1]]$shape
[1] 1.603642

[[1]]$rate
[1] 1.286430


[[2]]
     shape         rate 
  1.81991260   1.45992004 
 (0.02503046) (0.02309113)

##

Lassana KOITA 

E-mail: Lassana.Koita@aviation-civile.gouv.fr
http://www.stac.aviation-civile.gouv.fr/
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