Hi All,

I’m trying to run a gamma regression with time and time^2 as predictors. when only time but not time^2 are included, sampling is great. But once time^2 is included, it all falls apart. Any ideas?

data{

int Time2[126];

vector[126] LPS_concentration;

int Time[126];

int LPS[126];

int Subject[126];

}

parameters{

real<lower=0> a;

vector[18] a_Subject;

vector[2] b_Time;

real<lower=0> scale;

}

model{

vector[126] mu;

scale ~ exponential( 1 );

b_Time ~ normal( 0 , 0.1 );

a_Subject ~ normal( 0 , 0.25 );

a ~ gamma( 0.5/0.25 , 1/0.25 );

for ( i in 1:126 ) {

mu[i] = a + a_Subject[Subject[i]] + b_Time[LPS[i]] * Time[i] + b_Time2[LPS[i]]*Time2;

mu[i] = exp(mu[i]);

}

LPS_concentration ~ gamma( mu/scale , 1/scale );

}