For slope vector a, intercept c, pseudo-guessing parameter g,
upper bound u, and latent ability vector theta, the response probability
function is
$$\mathrm P(\mathrm{pick}=0|a,c,g,u,\theta) = 1- \mathrm P(\mathrm{pick}=1|a,c,g,u,\theta)
$$
$$\mathrm P(\mathrm{pick}=1|a,c,g,u,\theta) = g+(u-g)\frac{1}{1+\exp(-(a\theta + c))}
$$