What is testval in :

`mu = pm.Normal('mu', mu=178, sd=20, testval=df.height.mean())`

?

Using it gives me a very different posterior of mu.

What is testval in :

`mu = pm.Normal('mu', mu=178, sd=20, testval=df.height.mean())`

?

Using it gives me a very different posterior of mu.

Usually, we use the testval to

- infer/validate the shape of the random variable,
- set the starting value of the sampler

This is an indication of problem of your model, that it is sensitive to initial condition of the chain.