Convergence issues with measurement error model with rethinking

Hi, I’m having trouble with convergence in one of my models and wanted to ask if there’s something really obvious I’m doing wrong. I’m relatively new to Bayesian stats and haven’t posted here before so please forgive any errors and let me know if there’s something else I should include/do differently.

I have a dataset that looks like this:

group       	PC1 		k_mean	k_me
1     		1.9759287       7.0     1.0
2  		    -1.9759287      3.5     1.5
3   		-1.5508796      5.0     3.0
4   		0.8288762       6.0     2.0
5    		-1.1907181      5.0     3.0

The predictor is the first principal component from other continuous measures, and the outcome variable was originally scored as a single value from 1 to 8 for some groups or as ranges (e.g. 2-5, 3-4) or for others. I tried to incorporate the uncertainty originally captured by these ranges as measurement error (mostly following the implementation in Ch 15 of McElreath’s book where he models observed divorce rates across US states as distributed around the unknown true value with a standard deviation calculated from measurement error) (please also let me know if this is an obviously terrible way of doing this). I have tried running it without the measurement error and it works fine, so I assume it has something to do with my implementation there. I have tried it with simulated data where I knew the relationship between the predictor and outcome and it gave the same issues.

Here is my data list and model (using the rethinking package)

dlist <- list(
  k_obs = df$k_mean, #outcome variable ranging from 1 to 8
  k_me = df$k_me #error around outcome ranging from 0 to 3
  k_sd = df$k_me/sd(df$k_mean),
  p = standardize(df$PC1), #mean = 0, sd = 1
  N = nrow(df)

mod1 <- ulam(
    k_obs ~ dnorm(k_true , k_sd),
    vector[N]:k_true ~ dnorm(mu, sigma),
    mu <- a + bP*p,
    a ~ dnorm(4.5, 1.2),
    bP ~ dnorm(0, 1),
    sigma ~ dexp(1)
  ), control = list( adapt_delta = 0.99,
                      max_treedepth = 13),
  data=dlist, chains=4, cores=4, cmdstan=T)

I get the following messages:
"Chain 4 Informational Message: The current Metropolis proposal is about to be rejected because of the following issue:
Chain 4 Exception: normal_lpdf: Scale parameter is 0, but must be positive! (in ‘/var/folders/y1/jd6dgn9d12v5xfgbz7q5ttph0000gn/T/RtmpM5vJi8/model-ce86667fbf3.stan’, line 24, column 4 to column 34)
Chain 4 If this warning occurs sporadically, such as for highly constrained variable types like covariance matrices, then the sampler is fine,
Chain 4 but if this warning occurs often then your model may be either severely ill-conditioned or misspecified.
Chain 4

All 4 chains finished successfully.
Mean chain execution time: 6.4 seconds.
Total execution time: 6.6 seconds.
2000 of 2000 (100.0%) transitions hit the maximum treedepth limit of 10 or 2^10-1 leapfrog steps.
Trajectories that are prematurely terminated due to this limit will result in slow exploration.
Increasing the max_treedepth limit can avoid this at the expense of more computation.
If increasing max_treedepth does not remove warnings, try to reparameterize the model."

And these ESS and Rhats, which…don’t seem good:

> precis(mod1, depth=2)
                mean    sd  5.5% 94.5% n_eff   Rhat4
k_true[1]      -0.37  0.55 -0.94  0.22   2    1738654012688586
k_true[2]       0.27  1.15 -1.27  1.97   2    767807965779033
k_true[3]       0.19  1.15 -1.33  1.84   2    974503176613239
k_true[4]       0.08  0.94 -1.50  0.93   2    2752906058414171

Again, I’m new here so if I should have added additional/different (or less) information, or should have posted this elsewhere please do let me know. I did find some previous threads about measurement error but couldn’t figure out how to use the suggestions to fix my own model (partly because I’m not familiar with base Stan). I assume I’m not incorporating the uncertainty in my outcome measure in the model particularly well but am not sure how else to do it. Any help is much appreciated. Thanks!

1 Like

In case this helps- I also tried doing it in brms following the two examples by Solomon Kurz here. The first syntax with se() runs without errors and reasonable rhats, but the mi() syntax which seems closer to McElreath’s method gives the same message.

So this works ok as far as I can tell:

mymod_se <- 
  brm(data = dlist, family = gaussian,
      k_obs | se(k_sd, sigma=TRUE) ~ p,
      prior = c(prior(normal(4.5, 1.2), class = Intercept),
                prior(normal(0, 1), class=b),
                prior(exponential(1), class=sigma)), 
      iter = 2000, warmup = 1000, cores = 3, chains = 3,
      control = list(adapt_delta = 0.99,
                     max_treedepth = 13),

And this does not:

mymod_mi <- 
  brm(data = dlist, 
      family = gaussian,
      k_obs | mi(k_sd) ~ 1 + p,
      prior = c(prior(normal(4.5,1.2), class = Intercept),
                prior(normal(0,1), class=b),
                prior(exponential(1), class = sigma)),
      iter = 2000, warmup = 1000, cores = 4, chains = 4,
      control = list(adapt_delta = 0.99,
                     max_treedepth = 13),
      save_mevars = TRUE)

4000 of 4000 (100.0%) transitions hit the maximum treedepth limit of 13 or 2^13-1 leapfrog steps.
Trajectories that are prematurely terminated due to this limit will result in slow exploration.
Increasing the max_treedepth limit can avoid this at the expense of more computation.
If increasing max_treedepth does not remove warnings, try to reparameterize the model.
Warning message:
Parts of the model have not converged (some Rhats are > 1.05). Be careful when analysing the results! We recommend running more iterations and/or setting stronger priors. 

Again, any help is really appreciated!

1 Like

neat and well-written question!

I think what you describe is not the best way to approach the problem. I see two potentially better approaches:

  1. If the outcome variable can be assumed to be an interval scale (which looks unlikely), you could IMHO consider the data as interval censored. Similarly to left/right censoring discussed at 4.3 Censored data | Stan User’s Guide, the exact values can be integrated out and the likelihood becomes the difference of the CDF values at the bounds. Interval censoring is also supported by brms.

  2. If the outcome variable is in fact ordinal, I would model it as such (see e.g. for a tutorial). You’ll notice that for the cumulative ordinal model, the likelihood for a single value is computed by subtracting values corresponding to a threshold above and a threshold below the assumed “true value”, this can then be expanded to allow for ranges (i.e. interval censoring) - to get a likelihood for an interval, you just work with thresholds that are further apart. Ordinal models are supported by brms, although I am not sure that brms will let you have interval censoring with ordinal models.

In general explicit measurement error models as the one you describe can be hard to fit, so if you can avoid it, I would try avoiding it :-). I don’t immediately see why the model has problems, but inspecting the pairs plot including a and a subset of k_true is likely to provide some more insights.

Hope this makes sense and best of luck with your model!

1 Like

I’ll just add some to what @martinmodrak already said: the measurement models can be trouble to fit, and the chapter by @Solomon that you linked to also highlights this.

I notice that this implementation differs somewhat from the code used in the recoded Rethinking book. Specifically, this doesn’t use the 0 + intercept notation, using instead 1 + ... for the intercept. Later in the recoded chapter you linked, Dr. Kurz notes some nuance with how brms treats these intercept specifications and how it can affect what otherwise seem like reasonable priors. The other thing I noticed was that you haven’t passed any initial values to the model. Generally speaking, I find that Stan doesn’t need initial values to converge, but as noted, these models are tricky to estimate and so those initial values may help more than usual.

All this to say, if the model could be reparametrized, then you may have a better shot at getting a reliable estimate without all these errors. If you need the measurement error as currently conceived, then I’d see whether those couple of differences in your implementation help the model run (I’m not holding my breath, though). I agree that ultimately your outcome measures sounds likely ordinal, which may mean that even if you get a model to run it’s predictions may not align well with your observed data


Thanks so much for the responses and the suggestions! At some point I did try the initial values and the 0 + intercept notation (although my predictors are mean centered) and it didn’t seem to help. The outcome is technically ordinal so I will try modeling it that way based on the tutorial and see if I can figure out the interval censoring approach.

1 Like