biossay <- data.frame(doses=c(-.863,-.296,-.053,.727),
		       rats=c(5,5,5,5),deaths=c(0.5,1,3,4.5),freq=c(0.5,1,3,4.5)/5)

log.post<-function(alpha,beta,data=biossay){
  ldens <- 0
  for (i in 1:length(data$doses)){
    theta <- 1/(1+exp(-alpha-beta*data$doses[i]))
    ldens  <- ldens + data$deaths[i]*log(theta)+(data$rats[i]-data$deaths[i])*log(1-theta)
  }
  ldens
}


numIter <- 1000
alpha <- rep(0,numIter);
beta <- rep(0,numIter);

alpha[1] <- -5
beta[1] <- -5
e <- 1

for (i in 2:numIter) {
  alpha[i] <- runif(1,alpha[i-1]-e,alpha[i-1]+e);
  beta[i] <- runif(1,beta[i-1]-e,beta[i-1]+e);
  if (runif(1) > exp(log.post(alpha[i],beta[i])-log.post(alpha[i-1],beta[i-1]))) {
    alpha[i] <- alpha[i-1];
    beta[i] <- beta[i-1];
  }
}

plot(alpha,beta,type="l")