# The fit values is not the same as the parameters generating these data, does it mean the model not work?

Hi,

I am new to stan. Recently, I want to make sure my model works. So I generate some data points for stan model to recover. But it fails. So I wonder if the fitted parameter values are not similar to the parameter which generates the data, does it matter? Does it because of the impropriate prior information? Or does it because of the labeling degeneracy of mixture model? I am so confused, hope someone can help me out. Thanks!
My model is a simple Gaussian mixture model (with 4 gaussian distribution and 4 sita).

Below part is how I generate the synthetic data (it is written in python):

``````def ff(x,y,mux,muy,Dx,Dy,ro):
s=1./(2*np.pi*Dx*Dy*np.sqrt(1-ro**2))*np.exp(-1/(2*(1-ro**2))*((x-mux)**2/Dx**2+(y-muy)**2/Dy**2-2*ro*(x-mux)*(y-muy)/(Dx*Dy)))#stats.norm(loc=mu,scale=Dx).pdf(x)
return s

def fcont(x,y,vx,vy,Dx,Dy,ro,x0,y0,t):
k=0
mux = x0 + vx*t
muy = y0 + vy*t
sx = np.sqrt(2.0*Dx*t)
sy = np.sqrt(2.0*Dy*t)
mm = ff(x,y,mux,muy,sx,sy,ro)
k = k + mm#/len(s)#1/len(s)*ff(x,y,mux,muy,sx,sy,ro[i])
return k

def covmat(Dx,Dy,ro,t):
sx =  np.sqrt(2.0*Dx*t)+0.0001
sy =  np.sqrt(2.0*Dy*t)+0.0001
return [[sx**2,ro*sx*sy],[ro*sx*sy,sy**2]]

if __name__ == '__main__':

# generate field data
x0 = 0.
y0 = 0.
x01 = 0
y01 = 0
x02 = 0
y02 = 0
x03 = 0
y03 = 0
vxp = 1.
vyp = 2.
Dxp = 1.
Dyp= 1.
rop= 0.3
vxp2 = 2.
vyp2 = -0.5
Dxp2 = 0.3
Dyp2= 0.3
rop2= 0.1
vxp3 = -2.
vyp3 = -2.
Dxp3 = 0.2
Dyp3= 0.2
rop3= 0.1
vxp4 = -5
vyp4 = 4
Dxp4 = 0.3
Dyp4= 0.3
rop4= 0.2
st = 5.0
# Dx1 = np.array([10])
Count = 1
#s1 =  np.linspace(1,1,Count)
# print s1
position_est = []
#Urand = np.random.choice([0,1,2,3],size=200,p=[0.2,0.3,0.3,0.2])
#print Urand
#N = 1000
np.random.seed(10)
Urand = np.random.choice([0,1,2,3],size=10,p=[0.25,0.25,0.25,0.25])
np.random.seed(1)
rand = np.random.uniform(size=1)
#Urand = np.random.uniform(size=1000)
#rand.sample = np.repeat(Urand,N)
#Uinterval = np.linspace(0,1,Count)
for j in range(len(Urand)):
if Urand[j] == 0:
for U in rand:
i = int(U/(1./Count))
position = stats.multivariate_normal(mean=[x0+vxp*st,y0+vyp*st],cov=covmat(Dxp,Dyp,rop,st))
# position = stats.norm(loc=x0 + vxp*(st-s1[i]),scale=np.sqrt(2*Dxp*(st-s1[i])))
position_est.append(position.rvs())
elif Urand[j] == 1:
for U in rand:
i = int(U/(1./Count))
position = stats.multivariate_normal(mean=[x01 + vxp2*st,y01+vyp2*st],cov=covmat(Dxp2,Dyp2,rop2,st))
# position = stats.norm(loc=x0 + vxp*(st-s1[i]),scale=np.sqrt(2*Dxp*(st-s1[i])))
position_est.append(position.rvs())
elif Urand[j] == 2:
for U in rand:
i = int(U/(1./Count))
position = stats.multivariate_normal(mean=[x02 + vxp3*st,y02+vyp3*st],cov=covmat(Dxp3,Dyp3,rop3,st))
# position = stats.norm(loc=x0 + vxp*(st-s1[i]),scale=np.sqrt(2*Dxp*(st-s1[i])))
position_est.append(position.rvs())
else:
for U in rand:
i = int(U/(1./Count))
position = stats.multivariate_normal(mean=[x03 + vxp4*st,y03+vyp2*st],cov=covmat(Dxp4,Dyp4,rop4,st))
# position = stats.norm(loc=x0 + vxp*(st-s1[i]),scale=np.sqrt(2*Dxp*(st-s1[i])))
position_est.append(position.rvs())
position_est = np.array(position_est)
``````

Then my stan model is using the position data, and my stan model is below:

``````model='''
data {
int D; //number of dimensions
int K; //number of gaussians
int N; //number of data
int  M; // patch number at time
vector[D] y[N]; // observation data
vector[D] s;//oil spill location
matrix[N,M] st;
}

parameters {
simplex[K] theta; //mixing proportions
ordered[D] v[K];
ordered[D] Dif[K];
cholesky_factor_corr[D] L[K]; //cholesky factor of correlation matrix
}

transformed parameters{
cholesky_factor_cov[D,D] cov[K*M,N];
vector<lower=0>[D] sigma[K*M,N]; // standard deviations
vector[D] mu[K*M,N];
real<lower=-0.999> ro[K];
matrix[D,D] Omega[K];
matrix[D,D] Sigma[K*M,N];

for (k in 1:K) {
Omega[k] = multiply_lower_tri_self_transpose(L[k]);
ro[k]=Omega[k,2,1];
for (m in 1:M){
for (n in 1:N) {sigma[k+K*(m-1),n] = 0.05 + sqrt(2*(st[n][m])*Dif[k]);
mu[k+K*(m-1),n] = s+v[k]*st[n][m];
cov[k+K*(m-1),n] = diag_pre_multiply(sigma[k+K*(m-1),n],L[k]);
}
}
}

model {
real ps[K*M];
theta ~ dirichlet(rep_vector(2.0, K));
for(k in 1:K){
L[k] ~ lkj_corr_cholesky(2);// contain rho
}

for (n in 1:N){
for (k in 1:K){
for (m in 1:M){
ps[k+K*(m-1)] = log(theta[k])+multi_normal_cholesky_lpdf(y[n] | mu[k+K*(m-1),n], cov[k+K*(m-1),n]);
}
}
target += log_sum_exp(ps);
}
}
``````

The fit results are so unrealistic.

``````                  mean se_mean     sd   2.5%    25%    50%    75%  97.5%  n_eff   Rhat
theta[1]          0.49    0.02   0.03   0.44   0.46   0.49   0.51   0.55    4.0   1.07
theta[2]          0.21  7.9e-3   0.02   0.18    0.2    0.2   0.22   0.23    4.0   1.36
theta[3]          0.09  6.6e-3   0.01   0.06   0.08   0.09    0.1   0.11    5.0   1.12
theta[4]          0.22  5.6e-3   0.01    0.2   0.21   0.22   0.23   0.24    5.0   1.17
v[1,1]           -2.44    0.04   0.06  -2.57  -2.49  -2.42  -2.39  -2.36    3.0   2.32
v[2,1]          -29.56    0.22   0.53 -30.39 -30.02  -29.6 -29.09 -28.51    6.0   1.05
v[3,1]           -41.0    0.51    0.8 -42.11 -41.86 -40.71 -40.36 -39.86    2.0   2.99
v[4,1]          -65.84    0.41   0.82 -67.65  -66.5 -65.78 -65.24 -64.55    4.0   1.67
v[1,2]           -1.36    0.06   0.14  -1.65  -1.46  -1.33  -1.27  -1.01    6.0   1.66
v[2,2]         4.2e296     nan    inf1.3e2951.5e2962.8e2965.1e2961.5e297    nan    nan
v[3,2]             inf     nan    inf1.2e3069.7e3061.8e3072.7e3073.5e307    nan    nan
v[4,2]             inf     nan    inf2.1e3069.3e3061.8e3072.6e3073.5e307    nan    nan
Dif[1,1]         47.09     0.2   0.44  46.22  46.69  47.24  47.44  47.69    5.0   1.02
Dif[2,1]          9.82    0.05   0.14   9.49   9.76   9.83   9.94  10.08    8.0   1.07
Dif[3,1]         22.76    1.34   2.15  19.64  20.65  23.03  24.39  26.35    3.0   2.51
Dif[4,1]         34.96    1.35   2.78  28.69  34.21  35.77  36.93  38.47    4.0   1.19
Dif[1,2]         64.23    0.88   1.91  61.12  62.86  64.01  65.27  68.15    5.0    1.3
Dif[2,2]       2.8e284     nan    inf7.6e2824.3e2837.9e2833.5e2841.4e285    nan    nan
Dif[3,2]           inf     nan    inf1.6e3065.9e3069.7e3061.4e3071.8e307    nan    nan
Dif[4,2]           inf     nan    inf2.5e3065.2e3068.6e3061.3e3071.8e307    nan    nan
``````

Yes, it does matter. You also have problems with Rhat (with several values far from 1.0), and effective sample sizes in the single digits.

Unfortunately, it’s hard for me say where the problems with the model are. I’ve had one case where I had similar problems that went away when I used reasonable initial values for my parameters, but my model was highly nonlinear and had nothing to do with the kinds of models that seem to be common in statistics. Realistically, you’ve probably got a lot of work to do to debug your model.

Might want to take a look at this thread where Michael Betancourt imparts some of his wisdom on mixture models. He also links to one of his case studies in his first reply that might be worth looking at.

Oh thanks! I read the case studies and thought it is because of the labeling degeneracy of mixture model.

Hi I wonder is there requirement for minimum data number for the GMM model to work?