Hello,

I like to build a model, which uses Bayesian Inference to infer the true air speed `tas`

based on observations of the ground speed `gs`

and wind speed `wind`

.

For `gs`

, I can assume it to be Gaussian due to measurement error. Assuming a constant `sigma`

, I’ll get a model for this part like this:

```
import pymc as pm
import numpy as np
speeds = np.array([190.0, 181.2, 202.5])
with pm.Model() as model:
mean_gs = pm.Uniform("mean_gs", lower=0, upper=200)
sigma_gs = 2.0
gs = pm.Normal("gs", mu=mean_gs, sigma=sigma_gs, observed=speeds)
```

This is just based on the PyMC tutorial and Bayesian Methods for Hackers. It works as expected. The result is a posterior which represent probable values for `mu`

of the `gs`

Normal distribution.

Now I like to add `ws`

to obtain `tas = gs + wind`

as a distribution. (This is obviously a simplification from a physical standpoint, but ok for the demonstration here.)

I tried it like this:

```
with pm.Model() as model:
mean_gs = pm.Uniform("mean_gs", lower=0, upper=200)
sigma_gs = 5.0
mean_wind = pm.Uniform("mean_wind", lower=10, upper=40)
sigma_wind = 2.0
wind = pm.Normal('wind', mu=mean_wind, sigma=sigma_wind)
gs = pm.Normal("gs", mu=mean_gs, sigma=sigma_gs, observed=speeds)
tas = pm.Deterministic('tas', gs + wind)
```

But then the result are three different posteriors for `tas`

, because I have three observations (will be 5, with 5 observed and so on). If I’ll add observed values to `wind`

, they have to be the same dimension as the observed for `gs`

, otherwise I’ll get an error.

So obviously, this is not the right approach to my problem. However, I couldn’t find any examples for such use cases with contentious distributions and observations from multiple sources. How would I implement such problems with PyMC? Do I need three models, one for `gs`

, one for `ws`

and a third to merge the results of the first two?

Thank you for your thoughts!