mu = 1.8, sd = 1.3  for d=1, z=1
mu = 1.0, sd = 1.0  for d=0, z=1
mu = 2.2, sd = 1.3  for d=1, z=0
mu =   0, sd = 1.0  for d=0, z=0
drawn from ~N(mu,sd^2), where:
see fig4dat_init.do for generation of this dataset.
simulated data for figure 4, SJ article: Janes, Longton, Pepe. Accommodating       covariates in ROC analysis. 2008.