[R] pb with optimimization and fitdistr

[Nicolas Turenne]

Hello

i try to fit a data series (N below) with a model consisting of a 
mixture of two beta distributions
for that i am using fitdistr of package MASS

as follows

 > library(MASS)
 > N=c(796,3586,4089,3364,2745,1992,1120,432,99,10,0,0)
 >  d2 = (N-min(N)+0.000001)/(max(N)-min(N)+0.002)
 >  mixtBeta <- function(x,p,q,sh11,sh12,sh21,sh22) 
p*dbeta(x,shape1=sh11,shape2=sh12)+(q)*dbeta(x,shape1=sh21,shape2=sh22)
 >  ff= fitdistr(d2,mixtBeta , 
start=list(p=0.2,q=0.1,sh11=2,sh12=9,sh21=6,sh22=10))
Il y a eu 50 avis ou plus (utilisez warnings() pour voir les 50 premiers)
 > ff
        p              q             sh11           sh12           
sh21           sh22
  2.265173e+03   2.751748e+03   6.823009e-02   1.230362e-01   
7.130326e-01    1.069649e+00

but manually i find another set of parameters
p=0.26;q=0.12; sh11=2.1, sh12=9, sh21=5.8, sh22=9

S=c(0,1,2,3,4,5,6,7,8,9,10,11)/12
 N=c(796,3586,4089,3364,2745,1992,1120,432,99,10,0,0)
 d2 = (N-min(N)+0.001)/(max(N)-min(N)+0.002)
 x = c(1:100)/100
 p=0.26;q=0.12; y = p*dbeta( x, 2.1, 9  ) + q*dbeta( x, 5.8, 9  );
 plot(S,d2,xlim=c(0,1), ylim=c(0,1))
 lines(x, y )

Is there a problem with fitdistr if one uses a function with parameters ?
or a problem of initialization ?

thanks for help to understand.

regards
Nicolas


-- 
Nicolas Turenne - INRA, Unité Mathématique Informatique et Génome UR1077, F-78350 Jouy-en-Josas
http://genome.jouy.inra.fr/~turenne/nt.html