 # Suggestions for identifying summed random effects

I am trying to estimate a repeated measures model with individual-level random intercepts and correlated random slopes partitioned into genetic effects G and environmental effects E. The genetic effects are scaled with fixed row-wise relatedness matrix A, which is relatively sparse. The environmental effects are i.i.d. and meant to simply soak up any unobserved individual heterogeneity not captured by the fixed covariance genetic effects. The response model is specified by the sum G + E, which is making it difficult for identification of the distinct standard deviations and correlations of the G and E effects. The model does a somewhat good job of returning simulated SDs despite sampling quite poorly (with low bulk and tail ESS), but the correlations seem to jump around haphazardly.

I’ve experimented with implementing basic prior regularization and some constraints on the parameters, but I’m hoping I’ve missed more effective strategies for enhancing identification. My code is below. Note that I take advantage of a matrix normal parameterization of the genetic effects to avoid direct computation of the Kronecker product G %x% A.

``````data {
int<lower=1> N; //number of observations
int<lower=0> I; //number of individuals
int<lower=0> M; //number of genetic effects
int<lower=0> E; //number of environment effects
int<lower=1> id_j[N]; //index of individual observations
matrix[I,I] Amat; //genetic relatedness matrix
vector[N] X1; //covariate
real Y[N]; //normal response
}

transformed data{
matrix[I,I] LA = cholesky_decompose(Amat); //lower-triangle A matrix
real conditional_mean0 = 0; //fixed global intercept
}

parameters {
real<lower=-1,upper=1> conditional_mean1; //expectation for subject slopes
vector<lower=0>[M] sdG_U; //genetic effect SDs
vector<lower=0>[E] sdE_U; //environment effect SDs
real<lower=0> sd_R; //residual SD
cholesky_factor_corr[M] LG_u; //genetic effect correlations
cholesky_factor_corr[E] LE_u; //environment effect correlations
matrix[I,M] std_devG; //genetic deviations
matrix[I,E] std_devE; //environment deviations
}

transformed parameters {
matrix[I, M] G_re; //genetic random effects (G_intercept, G_slope)
matrix[I, E] E_re; //environment random effects (E_intercept, E_slope)
vector[I] P0; //phenotypic random intercepts (G + E)
vector[I] P1; //phenotypic random slopes (G + E)

//matrix normal sampling of genetic effects
G_re = LA * std_devG  * diag_pre_multiply(sdG_U, LG_u) ;
//environment effects
E_re = std_devE * diag_pre_multiply(sdE_U, LE_u) ;
//phenotypic effects (centered on global intercept and slope)
P0 = rep_vector(conditional_mean0, I) + col(G_re,1) + col(E_re,1);
P1 = rep_vector(conditional_mean1, I) + col(G_re,2) + col(E_re,2);
}

model{
conditional_mean1 ~ std_normal();
to_vector(std_devG)~std_normal();
to_vector(std_devE)~std_normal();
LG_u ~ lkj_corr_cholesky(3);
LE_u ~ lkj_corr_cholesky(3);
to_vector(sdG_U) ~ exponential(3);
to_vector(sdE_U) ~ exponential(3);
sd_R ~ exponential(3); //residual sd

//response Y ~ G_int + E_int + (G_slope + E_slope)*X1
Y ~ normal( P0[id_j] + P1[id_j].*X1, sd_R);
}
``````

Here are posterior plots indicative of the identification issues for the G and E SDs and correlations. I simulated G and E correlations of large magnitude and opposite sign (-0.5 and 0.5 respectively).

1 Like

Hi,
sorry for taking quite long to respond.

One thing I’ve found useful in a different context where I also had two sources of variability that were hard to distinguish was to have a parameter for total variability and then a proportion of the variability to be accounted for by one of the components, e.g. (for just one sd parameter):

``````parameters {
real<lower=0> total_sd;
real<lower=0,upper=1> varianceG_frac;
}

transformed parameters {
//simplified from sqrt((total_sd^ 2) * varianceG_frac)
real<lower=0> sdG = total_sd * sqrt(varianceG_frac);
//simplified sqrt((total_sd^ 2) * (1 - varianceG_frac))
real<lower=0> sdE = total_sd * sqrt(1 - varianceG_frac);
}
``````

Note that we need to transform from sd to variance, as variance is a linear operator while sd is not.

This way `total_sd` becomes usually well identified and independent of `varianceG_frac` (in the model I’ve used the posterior for this parameter was basically uniform over [0,1] indicating that we couldn’t learn the decomposition, so the model is effectively integrating it out - and the sampling worked quite well).

Does that make sense? And do you think this answers you inquiry?

Not sure if you could decompose the whole covariance matrix this way, but it might be possible… (my linear algebra is too rusty for that). Or maybe just doing this for the standard deviations would be enough.

Best of luck with your model!

2 Likes

Thanks Martin, this is a great idea that had not occurred to me. It seems quite sensible in this context. I will give it a shot and report back!

A brief note as well for anyone who visits this topic in the mean time. The code above is missing transpose operations on the matrix normal parameterization of the random effects.

`````` //matrix normal sampling of genetic effects
G_re = LA * std_devG  * diag_pre_multiply(sdG_U, LG_u) ;
//environment effects
E_re = std_devE * diag_pre_multiply(sdE_U, LE_u) ;
``````

Should be

`````` //matrix normal sampling of genetic effects
G_re = LA * std_devG  * diag_pre_multiply(sdG_U, LG_u)' ;
//environment effects
E_re = std_devE * diag_pre_multiply(sdE_U, LE_u)';
``````

I’ll post updated code once I’ve attempted Martin’s suggestion.

1 Like