model {
tau ~ dgamma(20, 2000)
sigma <- 1/sqrt(tau)
theta ~ dnorm(5, 0.25)
n <- 2*pow((1.28 + 1.96)*sigma/theta, 2) # n for 90% power
power <- phi(sqrt(84/2)*theta/sigma - 1.96) # power for n = 84
p70 <- step(power - 0.7) # Pr(power > 70%)
}

   node   mean   sd   MC error   2.5%   median   97.5%   start   sample
   n   38740.0   2.533E+6   25170.0   24.73   87.93   1487.0   1   10000
   p70   0.7012   0.4577   0.004538   0.0   1.0   1.0   1   10000
   power   0.7739   0.2605   0.002506   0.1151   0.8863   1.0   1   10000
   
[example-5_3_1-power0][example-5_3_1-power1]