Jesús Humberto Gómez writes:
I am an epidemiologist and currently I am studying my fourth year of statistics degree.
Currently we have a dataset with data structure shown here:
We want to investigate the effect of mining contamination on the blood lead levels. We have a total of 8 inhabited locations and the participants and politicians want to know the mean levels in each location.
To give an answer, we have constructed a hierarchical model: level one children (or mothers), level two defined by the locations and mining zone like a population effect (what is, a fixed effect). There are no explainatory variables at the second level. The sizes of the locations are 114, 37, 19, 11, 63, 56, 40, 12 (first four mining zone, second four non mining zone).
The model converges properly. Our data are left censored data and this has been taken account in the likelihood. The mining effect obtained is 23% higher that the obtained running the model without the hierarchical structure (0.59 vs 0.48) and this worries us.
In addition, we have doubts about what we are doing arise because locations are nested in the zones (mining vs non mining) and we are modeling it like a population effect.
Then, is this approach correct?
At the moment, we are going to study separately the mining effect in children and mothers, but in the future we will study both mothers and children together since the blood lead levels correlation is high.
Of course, we are using Stan.
First, good call on using Stan! This gives you the flexibility to expand the model as needed.
Now on to the question. Without looking at all the details, I have a few thoughts:
First, if you fit a different model you’ll get a different estimate, so in that sense there’s no reason to be bothered that your estimate is changing.
But it does make sense to want to understand why, or should I say how, the estimate changes when you add more information, or when you improve your model. For simple regressions there are methods such as partial correlations that are designed to facilitate these explanations. We need something similar for multilevel models—for statistical models in general—a “trail of breadcrumbs” tracking how inferences for qois change as we change our models.
For the particular example discussed above, I have one more suggestion which is to include, as a group-level predictor, the group-level average of your individual-level predictor. Bafumi and I discuss this in our unpublished paper from 2006.