Persistent marijuana use : evidence from the NLSY

We analyze persistence in marijuana consumption utilizing data from the 1997 cohort of the National Longitudinal Survey of Youth (NLSY97). We allow for three sources of persistence: pure state dependence, time invariant unobserved heterogeneity and persistence in idiosyncratic, time-varying shocks. We also consider intensity of consumption based on days of use per month and estimate a dynamic ordered Probit model using simulated Maximum Likelihood. We consider a Polya model that generalizes the more commonly used Markov models. The results show that there is a causal eect of previous use. However, ignoring unobserved heterogeneity and serially correlated shocks signicantly exaggerates the state dependence.