Initial values for nodes in data{} block

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Aziz
2011-09-12
2012-09-01
  • Aziz
    Aziz
    2011-09-12

    Hi,

    is there any way to provide initial values for stochastic nodes given in the
    data{} block (i.e. nodes for which we want to fix the sampling distribution)?

    I need to do this because my model uses a dinterval() construct which requires
    adequate initial values to work. The sampling distribution of some of the
    parameters is known so I wish to provide these in the data block so that these
    do not get updated by likelihood information from the data. My initial idea
    was to avoid the data{} block and provide the sampling distribution in the
    model{} block. I then locked these distribution with dsum() which mimics the
    use of the cut() function in BUGS. However as known dsum() used in such
    fashion (or cut()) do not actually provide the correct distribution so it is
    not a solution. So Im looking at a way to provide initial values for nodes in
    the data{} block. Is there any such possibility in JAGS?

    Thanks

     
  • Aziz
    Aziz
    2011-09-12

    It seems that stochastic nodes in the data block are forward sampled only once
    (and not once per iteration...) so it is not likely to work. The cut function
    seems to work in WinBUGS for my problem but I'd like to use JAGS as it
    provides some other features that WinBUGS doesn't have. For now I tried to
    implement the cut function in JAGS this way (suppose the sampling distribution
    of alpha is known with mean 10 and sd=sqrt(1/0.5)):

    alpha ~ dnorm(10,0.5)

    alpha.cut ~ dsum(alpha)

    for(i in 1:N){

    y_ ~ dnorm(alpha.cut*x_,tau)

    }

    tau <- pow(sigma,-2)

    sigma ~ dunif(0,1000)

    However in this fashion dsum doesn't have any effect (alpha gets modified by
    the likelihood of y). How should I use dsum() so that alpha doesn't get
    updated by y?__