# Defining specific eta's

Instead of writing with MvNormal, I am using Normal in the @random block. Please check is this code is correct.

``````    @param begin
tvcl     ∈ RealDomain(lower=0)
tvvc     ∈ RealDomain(lower=0)
Ω        ∈ PDiagDomain(2)
end

@random begin
ηCL        ~ Normal(0, Ω)
ηVc        ~ Normal(0, Ω)

end
``````

Welcome to the discussion board @jayashreed1412 !

while the specification of the `@random` block is correct, you need to adjust the `@param` block. Let me show the two ways of writing the same model.

### Using `PDDiagDomain` for your variances.

In this case, the `η`'s are sampled from a `MvNormal` distribution of `Ω` which is defined as Positive Definite Diagonal (PDDiag) Matrix Domain. The number inside the parenthesis of `PDDiagDomain` represents the number of diagonal elements (the number `η`'s ) in the block.
Important Note: The `Ω` here represents a variance for the `MvNormal`.

``````    @param begin
tvcl     ∈ RealDomain(lower=0)
tvvc     ∈ RealDomain(lower=0)
Ω        ∈ PDiagDomain(2)
end

@random begin
η ~ MvNormal(Ω)
end
``````

When you write the model this way, the `η`'s are a vector and hence have to be indexed to access. So, your `@pre` block would like this where `η[1]` and `η[2]` below represent the samples from diagonals of the matrix.

``````    @pre begin
CL = tvcl * exp(η[1])
Vc = tvvc * exp(η[2])
end
``````

### Using `RealDomain` for your variances

In this case, the `η`’ for each parameter is sampled from a `Normal` distribution of `ω` which is defined as Real Domain which works like any other parameter. Each `η` will have its corresponding `ω`.
Important Note: The `ω` here is the standard deviation of the `Normal` distribution.

``````    @param begin
tvcl     ∈ RealDomain(lower=0)
tvvc     ∈ RealDomain(lower=0)
ωCL      ∈ RealDomain(lower=0)
ωVc      ∈ RealDomain(lower=0)
end

@random begin
ηCL        ~ Normal(0.0, Ω)
ηVc        ~ Normal(0.0, Ω)
end
``````

When you write the model this way, the `η` can be named (e.g. `ηCL`) which is easier to read and remember than an indexed version. So, your `@pre` block would like this where `ηCL` and `ηVc` below represent the samples from a `Normal` distribution with mean `0.0` and variance `ω` (Note mean should be `0.0` and not `0`)

``````    @pre begin
CL = tvcl * exp(η[1])
Vc = tvvc * exp(η[2])
end
``````

@vijay It is working thank you.