duration_model.py

with pm.Model() as duration_model:
    # Three parameters to sample
    alpha_skew = pm.Normal('alpha_skew', mu=0, tau=0.5, testval=3.0)
    mu_ = pm.Normal('mu', mu=0, tau=0.5, testval=7.4)
    tau_ = pm.Normal('tau', mu=0, tau=0.5, testval=1.0)
    
    # Duration is a deterministic variable
    duration_ = pm.SkewNormal('duration', alpha = alpha_skew, mu = mu_, 
                              sd = 1/tau_, observed = duration)
    
    # Metropolis Hastings for sampling
    step = pm.Metropolis()
    duration_trace = pm.sample(N_SAMPLES, step=step)