Estimation of models with zero left-censored variables. Null values may be caused by a selection process Cragg (1971) <doi:10.2307/1909582>, insufficient resources Tobin (1958) <doi:10.2307/1907382> or infrequency of purchase Deaton and Irish (1984) <doi:10.1016/0047-2727(84)90067-7>.
