# Model fitting and sampling issue: Only 1 chain sampling properly

Hi all,

I’ve been here a lot lately, so I’m very sorry about that!

Thanks to the community, I was able to get my model running, but noticed something strange in the output… So, I was running 4 chains with 2000 iterations. I noticed that chains 2, 3, and 4 were running in synchrony, and chain 1 was lagging behind. When the model finished running, I was told the following:

Warning message:
In writable_sample_file(diagnostic_file) :Warning message:
Warning message:
1: There were 8 divergent transitions after warmup. Increasing adapt_delta above 0.8 may help. See
http://mc-stan.org/misc/warnings.html#divergent-transitions-after-warmup

"/Users/mjarviscross/Desktop/GitHub/FcnalRespProj/data/Type II/FcnalRespProj" is not writable; use "/var/folders/l_/xb7lfbls12nb0dwxdrww8gn40000gn/T//RtmpDSeoCR" instead
In writable_sample_file(diagnostic_file) :In writable_sample_file(diagnostic_file) :Warning message:

In writable_sample_file(diagnostic_file) :2: There were 3963 transitions after warmup that exceeded the maximum treedepth. Increase max_treedepth above 10. See
http://mc-stan.org/misc/warnings.html#maximum-treedepth-exceeded

"/Users/mjarviscross/Desktop/GitHub/FcnalRespProj/data/Type II/FcnalRespProj" is not writable; use "/var/folders/l_/xb7lfbls12nb0dwxdrww8gn40000gn/T//RtmpIJCP4C" instead
"/Users/mjarviscross/Desktop/GitHub/FcnalRespProj/data/Type II/FcnalRespProj" is not writable; use "/var/folders/l_/xb7lfbls12nb0dwxdrww8gn40000gn/T//Rtmp7jEFf6" instead
3: There were 1 chains where the estimated Bayesian Fraction of Missing Information was low. See
http://mc-stan.org/misc/warnings.html#bfmi-low
4: Examine the pairs() plot to diagnose sampling problems

"/Users/mjarviscross/Desktop/GitHub/FcnalRespProj/data/Type II/FcnalRespProj" is not writable; use "/var/folders/l_/xb7lfbls12nb0dwxdrww8gn40000gn/T//RtmphaWosk" instead
5: The largest R-hat is 5.43, indicating chains have not mixed.
Running the chains for more iterations may help. See
http://mc-stan.org/misc/warnings.html#r-hat
6: Bulk Effective Samples Size (ESS) is too low, indicating posterior means and medians may be unreliable.
Running the chains for more iterations may help. See
http://mc-stan.org/misc/warnings.html#bulk-ess
7: Tail Effective Samples Size (ESS) is too low, indicating posterior variances and tail quantiles may be unreliable.
Running the chains for more iterations may help. See
http://mc-stan.org/misc/warnings.html#tail-ess


And when I looked at the trace plots, I noticed that only the first chain seemed to be sampling properly:

This seems like a buggy problem rather than a conceptual problem, so I was wondering if anyone’s seen something like this before, and if anyone knows how to amend the situation.

Here’s my model (in rstan):

library(rstan)
library(gdata)
library(bayesplot)
library(tidyverse)
library(brms)

write("// Stan Model;
functions{
real[] dZ_dt(
real t, // Time
real[] Z, // System state {Parasite, Host}
real[] theta, // Parms
real[] x_r, // Real data; below is integer data
int[] x_i){
real P = Z[1]; // System state coded as an array, such that Z = (P,H)
real H = Z[2];

real r = theta[1];  // Parameters of the system, in order they appear
real O = theta[2];
real h = theta[3];
real b = theta[4];
real c = theta[5];
real u = theta[6];

real dP_dt = P*r - H*(O*P/1 + O*P*h); // Mechanistic model
real dH_dt = b + H*(c*(O*P/1 + O*P*h)-u);
return({dP_dt,dH_dt}); // Return the system state
}
}

data{
int<lower=0>N; // Define N as non-negative integer
real ts[N]; // Assigns time points to N
int y_init[2]; // Initial conditions for ODE
int<lower=0>y[N,2]; // Define y as real and non-negative
}

parameters{
real<lower=0>r;
real<lower=0,upper=1>O;
real<lower=0>h;
real<lower=0>b;
real<lower=0>c;
real<lower=0,upper=1>u;
real<lower=0>Z_init[2]; // Initial population size non-neg
}

transformed parameters{
// Stiff solver for ODE
real Z[N,2]
= integrate_ode_bdf(dZ_dt,Z_init,0,ts,{r, O, h, b, c, u},rep_array(0.0,2),rep_array(0,2),1e-10,1e-10,2e4);
for (i in 1:N) {
Z[i,1] = Z[i,1] + 1e-6;
Z[i,2] = Z[i,2] + 1e-6;
}
}

model{ // Ignore the means/variances for now...
r~normal(2.5,1); // r
O~beta(2,2); // O; bounded between 0 and 1
h~normal(0.06,1); // h
b~normal(35,1); // b
c~normal(0.2,1); // c
u~beta(2,2); // u; bounded between 0 and 1
Z_init~uniform(0,400);
for (k in 1:2){
y_init[k]~poisson(Z_init[k]);
y[ ,k]~poisson(Z[ ,k]);
}
}",
"Stan_Model_TypeII.stan")

stanc("Stan_Model_TypeII.stan") # To check that we wrote a file (I think we did?)

Stan_Model_TypeII <- stan_model("Stan_Model_TypeII.stan")

# Squeezing the data into a form that Stan gets
N <- length(Stoch_Data_TypeII$t)-1 # df is 2923; N is 2922 ts <- 1:N y_init <- c(10,350) # Initial states, P = 10; H = 350 y <- as.matrix(Stoch_Data_TypeII[2:(N+1),3:4]) y <- cbind(y[ ,2],y[ ,1]); # This worked, sick; where y[,1] is H, and y[,2] is P Stan_StochData_TypeII <- list(N=N,ts=ts,y_init=y_init,y=y) # // Make non-neg real<lower=0>alpha[{1,3,4,5}]; # // Proportions bounded between 0 and 1 real<lower=0,upper=1>alpha[{2,6}] # Fitting the data to the model Stan_Model_TypeII <- stan_model("Stan_Model_TypeII.stan") fit2 <- sampling(Stan_Model_TypeII, data = Stan_StochData_TypeII, warmup = 1000, iter = 2000, chains = 4, cores = 4, thin = 1, check_data = TRUE, diagnostic_file = "/Users/mjarviscross/Desktop/GitHub/FcnalRespProj/data/Type II/FcnalRespProj/TypeII_Fitting_Output2.R", seed = 124, show_messages = TRUE, verbose = TRUE) #### Tracking progress #### # 1st run (1 chain, 500 iter) #### 806.348 seconds (Warm-up)--13.5 min #### 2514.75 seconds (Sampling)--42 min #### Rhat 1.71 #### 233 transitions after warmup that exceeded the maximum treedepth; increase max_treedepth above 10 #### Bulk Effective Samples Size (ESS) is too low OutputSum_1 <- fit fit1_plot <- plot(fit) fit1_trace <- traceplot(fit) # 2nd run (4 chains, 2000 iter) #### Chain 1, 173 min warm/357 min samp; Chain 2, 92 min warm/230 min samp; Chain 3, 88 min warm/242 min samp; Chain 4, 87 min warm/253 min samp #### Rhat 5.43 #### 8 divergent transitions after warmup; ncreasing adapt_delta above 0.8 may help #### 3963 transitions after warmup that exceeded the maximum treedepth; increase max_treedepth above 10 #### Bulk Effective Samples Size (ESS) is too low; tail Effective Samples Size (ESS) is too low--more iterations! #### 1 chains where the estimated Bayesian Fraction of Missing Information was low fit2_plot <- plot(fit2) fit2_trace <- traceplot(fit2)  I would appreciate any advice people have! Thank you all so much for putting up with me! :) 1 Like I’ve been here a lot lately, so I’m very sorry about that! You shouldn’t feel sorry about that. I know next to nothing about ODEs. So, I’ll stick to very generic comments. • The traceplots suggest to me that the model is not well identified and the sampler gets stuck. In other words, I think this might still be a conceptual issue. • A quick look at the main part of the ODE shows that there are a lot of multiplications, partly between parameters. These type of multiplications are often hard to identify.  real dP_dt = P*r - H*(O*P/1 + O*P*h); // Mechanistic model real dH_dt = b + H*(c*(O*P/1 + O*P*h)-u);  • The only healthy looking chain is chain 1 for Z[1,1]. So, my take-away would be that there are too many parameters that determine the outcome of Z[1,1], so that Z[1,1] is potentially identified by the data but the parameters are not. Your first step could be to simplify the underlying model even more by either fixing some parameters to one value or by decreasing the number parameters. Good luck with the model and hopefully my response bumps your post up so that knowledgeable people can have a look at your model. 2 Likes Hi @stijn, thanks so much for your comment! Unfortunately, I can’t simplify the model without losing its representation of the system… It seems like this model may be sensitive to initial conditions, so I was thinking giving the model the initial state of the system, as opposed to having it estimate Z[1,1] and Z[1,2], could help, which kind of speaks to your third point. Unfortunately, I’ve tried to implement that, and now, running two chains, the second chain runs all the way through, pretty quickly, and the first chain stagnates at 0%…  functions{ real[] dZ_dt( real t, // Time real[] Z, // System state {Parasite, Host} real[] theta, // Parms real[] x_r, // Real data; below is integer data int[] x_i){ real P = Z[1]; // System state coded as an array, such that Z = (P,H) real H = Z[2]; real r = theta[1]; // Parameters of the system, in order they appear real O = theta[2]; real h = theta[3]; real b = theta[4]; real c = theta[5]; real u = theta[6]; real dP_dt = P*r - H*(O*P/1 + O*P*h); // Mechanistic model real dH_dt = b + H*(c*(O*P/1 + O*P*h)-u); return({dP_dt,dH_dt}); // Return the system state } } data{ int<lower=0>N; // Define N as non-negative integer real ts[N]; // Assigns time points to N real y_init[2]; // Initial conditions for ODE int<lower=0>y[N,2]; // Define y as real and non-negative } parameters{ real<lower=0>r; real<lower=0,upper=1>O; real<lower=0>h; real<lower=0>b; real<lower=0>c; real<lower=0,upper=1>u; } transformed parameters{ // Stiff solver for ODE real Z[N,2] = integrate_ode_bdf(dZ_dt,y_init,0,ts,{r, O, h, b, c, u},rep_array(0.0,2),rep_array(0,2),1e-10,1e-10,2e4); for (i in 1:N) { Z[i,1] = Z[i,1] + 1e-6; Z[i,2] = Z[i,2] + 1e-6; } } model{ // Ignore the means/variances for now... r~normal(2.5,1); // r O~beta(2,2); // O; bounded between 0 and 1 h~normal(0.06,1); // h b~normal(35,1); // b c~normal(0.2,1); // c u~beta(2,2); // u; bounded between 0 and 1 for (k in 1:2){ y[ ,k]~poisson(Z[ ,k]); } }  And the r script: # Squeezing the data into a form that Stan gets Stoch_Data_TypeII = read.csv('/Users/mjarviscross/Desktop/GitHub/FcnalRespProj/data/Type II/FcnalRespProj_Data/Stoch_Data_TypeII.csv') N <- length(Stoch_Data_TypeII$t)-1 # df is 2923; N is 2922
ts <- 1:N
y_init <- c(10,350) # Initial states, P = 10; H = 350
y <- as.matrix(Stoch_Data_TypeII[2:(N+1),3:4])
y <- cbind(y[ ,1],y[ ,2]); # This worked, sick; where y[,1] is P, and y[,2] is H
Stan_StochData_TypeII <- list(N=N,ts=ts,y_init=y_init,y=y)

# Fitting the data to the model
Stan_Model_TypeII <- stan_model("Stan_Model_TypeII.stan")
fit3 <- sampling(Stan_Model_TypeII,
data = Stan_StochData_TypeII,
warmup = 200,
iter = 400,
chains = 2,
cores = 2,
thin = 1,
# control = list(max_treedepth=15,
check_data = TRUE,
diagnostic_file = "/Users/mjarviscross/Desktop/GitHub/FcnalRespProj/Fit3.csv",
show_messages = TRUE,
verbose = TRUE)


Sorry to hear that. Your problem is far outside my comfort zone. I am not sure I can help you.

If I am right about the identifiability (big if!), then that would mean that the system has too many free parameters, to be estimated from the data. One way to test this is to see whether you can recover data that is generated based on your priors and model.

1 Like

This cannot be correct.

real dP_dt = P*r - H*(O*P/1 + O*P*h); // Mechanistic model
real dH_dt = b + H*(c*(O*P/1 + O*P*h)-u);


Remember the operator precedence rules. Division happens before addition. If the mechanistic model is meant to be \frac{dP}{dt} = Pr + H\times\frac{OP}{1+hOP} then the denominator must be enclosed in parenthesis. I would also combine common subexpressions.

real OP = O*P;
real OP_h = OP / (1 + h*OP);
real dP_dt = P*r - H*OP_h; // Mechanistic model
real dH_dt = b + H*(c*OP_h-u);

3 Likes

I’ll try that––thanks!

I think you could be right about this! I’m trying to feed the model the initial values rather than having it estimate them, but I think I’m messing something up with the code…

Can you give the mechanistic model in mathematical notation? That might help us check whether Niko is right.

It should be as follows:

I’ve implemented some changes, including @nhuurre’s, and it looks like identifiability may be a big part of the issue; the model usually fits, but the chains don’t converge, and convergence gets worse as I add more iterations and more chains.

I’m also trying to add a generated quantities block, which is proving problematic. Do you have any advice?

Here’s watch I’m working with now:

write("// Stan Model;
functions{
real[] dZ_dt(
real t, // Time
real[] Z, // System state {Parasite, Host}
real[] theta, // Parms
real[] x_r, // Real data; below is integer data
int[] x_i){
real P = Z[1]; // System state coded as an array, such that Z = (P,H)
real H = Z[2];

real r = theta[1];  // Parameters of the system, in order they appear
real O = theta[2];
real h = theta[3];
real b = theta[4];
real c = theta[5];
real u = theta[6];

real FR_II = O*P/(1 + O*P*h);
real dP_dt = P*r - H*FR_II; // Mechanistic model
real dH_dt = b + H*(c*FR_II-u);
return({dP_dt,dH_dt}); // Return the system state
}
}

data{
int<lower=0>N; // Define N as non-negative integer
real ts[N]; // Assigns time points to N
real y0[2]; // Initial conditions for ODE
real t0; // Initial time point
int<lower=0>y[N,2]; // Define y as real and non-negative
}

parameters{
real<lower=0>r;
real<lower=0,upper=1>O;
real<lower=0>h;
real<lower=0>b;
real<lower=0>c;
real<lower=0,upper=1>u;
}

transformed parameters{
// Stiff solver for ODE
real Z[N,2]
= integrate_ode_bdf(dZ_dt,y0,t0,ts,{r, O, h, b, c, u},rep_array(0.0,2),rep_array(0,2),1e-10,1e-10,2e4);
for (i in 1:N) {
Z[i,1] = Z[i,1] + 1e-6;
Z[i,2] = Z[i,2] + 1e-6;
}
}

model{ // Ignore the means/variances for now...
r~normal(2.5,1); // r
O~beta(2,2); // O; bounded between 0 and 1
h~normal(0.06,1); // h
b~normal(35,1); // b
c~normal(0.2,1); // c
u~beta(2,2); // u; bounded between 0 and 1
for (k in 1:2){
y[ ,k]~poisson(Z[ ,k]);
}
}",
"Stan_Model_TypeII.stan")

stanc("Stan_Model_TypeII.stan") # To check that we wrote a file

Stan_Model_TypeII <- stan_model("Stan_Model_TypeII.stan")

# Squeezing the data into a form that Stan gets
N <- length(Stoch_Data_TypeII\$t)-1 # df is 2923; N is 2922
ts <- 1:N
y_init <- c(10,350) # Initial states, P = 10; H = 350
y0 <- array(y_init) # For the ODE solver
t0 <- 0.0
y <- as.matrix(Stoch_Data_TypeII[2:(N+1),3:4])
y <- cbind(y[ ,1],y[ ,2]); # This worked, sick; where y[,1] is P, and y[,2] is H
Stan_StochData_TypeII <- list(N=N,ts=ts,y0=y0,t0=t0,y=y)

# Fitting the data to the model
Stan_Model_TypeII <- stan_model("Stan_Model_TypeII.stan")
fit5 <- sampling(Stan_Model_TypeII,
data = Stan_StochData_TypeII,
warmup = 200,
iter = 400,
chains = 2,
cores = 2,
thin = 1,
control = list(max_treedepth=15,
seed = 123,
check_data = TRUE,
diagnostic_file = "/Users/mjarviscross/Desktop/GitHub/FcnalRespProj/Fit5.csv",
show_messages = TRUE,
verbose = TRUE)


And the generated quantities block I want to add looks like this:

generated quantities {
int y_rep[N, 2];
for (k in 1:2) {
for (n in 1:N)
y_rep[n, k] = poisson_rng(Z[ ,k]);
}
}


But am getting the following error:

SYNTAX ERROR, MESSAGE(S) FROM PARSER:
Dimension mismatch in assignment; variable name = y_rep, type = int; right-hand side type = int[ ].
Illegal statement beginning with non-void expression parsed as
y_rep[[n, k]]
Not a legal assignment, sampling, or function statement.  Note that
* Assignment statements only allow variables (with optional indexes) on the left;
* Sampling statements allow arbitrary value-denoting expressions on the left.
* Functions used as statements must be declared to have void returns

1 Like

This line probably needs to be

y_rep[n, k] = poisson_rng(Z[n,k]);


The syntax error message is saying that y_rep[n,k] is an integer while poisson_rng(Z[,k]) is an array of integers and those can not be assigned to each other.

As a sketch for how I think you can look for non-identifiable parameters see below. But this is probably not a useful way of thinking about it, I just write it down as how I tried to help but probably failed. You really want someone who actually knows how ODEs work. I just put my simplistic economist hat on and look at what would happen if the system stabilises. That is \frac{dP}{dt} = 0 and \frac{dH}{dt} = 0. If you rewrite everything a little bit you can then write H as H = (b - r) \times f(P, \theta, c, h, \mu) where f is not a function of b or r. That means that if(!) most of the data is from observations close to a stable system b and r are poorly identified but their difference is well defined. In my simpler models I would then reparametrise (b, r) to (\Delta = b - r, M = b/2 + r/2) which sometimes works.

1 Like

That’s a really interesting idea! Thank you so much––I really appreciate all of your help! :)