FindDistributionParameters for a ParameterMixtureDistribution

Question

Let's say I define a simple hierarchical model as follows (I don't actually particularly care about the distributions chosen):

modelDist = ParameterMixtureDistribution[NormalDistribution[a, 1], 
a \[Distributed] GammaDistribution[b, 1], Assumptions -> b > 0]

I then generate vals, a list of random samples from a version of this distribution where the true value of b is 2.

If I do

FindDistributionParameters[vals, 
 ParameterMixtureDistribution[NormalDistribution[a, 1], 
  a \[Distributed] GammaDistribution[b, 1], Assumptions -> b > 0]]

I get error messages from NMaximize that suggest it's trying to fit both b and a, even though a is just a dummy variable. If I provide an initial value for b,

FindDistributionParameters[vals, 
 ParameterMixtureDistribution[NormalDistribution[a, 1], 
  a \[Distributed] GammaDistribution[b, 1], 
  Assumptions -> b > 0], {{b, 1.}}]

I get the message

FindDistributionParameters::prms: The parameters to be estimated in ParameterMixtureDistribution[NormalDistribution[a,1],a\[Distributed]GammaDistribution[b,1],Assumptions->b>0] are not the same as the parameters in {{b,1.}}.

Is there a way to make this work? Specifically, is there a way to get FindDistributionParameters to recognize that a isn't actually a parameter that needs to be fit? Perhaps some kind of symbol manipulation or evaluation tricks?

Answer 1

I got this to work by deleting the Assumptions on b:

FindDistributionParameters[
    vals, 
    ParameterMixtureDistribution[
       NormalDistribution[a, 1], 
       a \[Distributed] GammaDistribution[b, 1]
    ]
]