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Kyle Bogdan Grant Brown. A (poor) Gibbs Sampling Approach to Logistic Regression. Data. Simulated based on known values of parameters (one covariate, ‘dose’). ‘rats’ given different dosages of imaginary chemical, 4 dose groups with 25 rats in each group.

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Kyle bogdan grant brown

Kyle Bogdan

Grant Brown

A (poor) Gibbs Sampling Approach to Logistic Regression


A poor gibbs sampling approach to logistic regression
Data

  • Simulated based on known values of parameters (one covariate, ‘dose’).

  • ‘rats’ given different dosages of imaginary chemical, 4 dose groups with 25 rats in each group.

  • Data generated three times under different parameters, three chains used for each data set.


Gibbs sampling for logistic data
Gibbs Sampling For Logistic Data?

  • Traditionally, binomial likelihood, prior on logit.

  • Full Conditionals have no coherent form.

  • Attractive, however, because it eliminates the need to reject iterations


Algorithm
Algorithm

  • Groenewald and Mokgatlhe, 2005

  • Create Uniform Latent Variables Based on Y[i,j] = 0, 1

  • Draws from joint posterior of Betas and U[i,j]

    • pi[i] = p(uniform(01) <= logit-1(Beta*x[i]))

  • Written in R, refined in Python

  • Very inefficient

    • Draw new parameter for each Y[i,j] at each iteration


Implementation
Implementation

  • Three datasets

  • Three chains per set

  • 1 Million iterations per chain

  • Last 500k iterations sent to CODA

  • 9m total iterations, 4.5 m analyzed








Winbugs model
WinBUGS Model

  • Y[i,j]’s given binomial (instead of Bernoulli) likelihood

  • Betas regressed on logit of proportion

  • Locally uniform priors on beta1 and beta2


Winbugs model1
WinBUGS Model

model{ for (i in 1:N){ r[i] ~ dbin(p[i], n[i]);logit(p[i]) <- (beta1 + beta2*(x[i] - mean(x[]))); r.hat[i] <- (p[i] * n[i]);} beta1 ~ dflat(); beta2 ~ dflat(); beta1nocenter <- beta1 - beta2*mean(x[]);}






Winbugs wins
WinBUGS Wins

  • Uses proportions instead of Individual Y[i,j]’s

  • Convergence is Better

  • WinBUGS appears more precise (more trials needed)

  • Also, much faster.


Resources
Resources

  • Groenewald, Pieter C.N., and Lucky Mokgatlhe. "Bayesian computation for logistic regression.“ Computational Statistics & Data Analysis 48 (2005): 857-68. Science Direct. Elsevier. Web. <http://www.sciencedirect.com/>.

  • Professor Cowles