This paper provides a ﬁrst assessment of a statistical dialog system in public use. In our dialog system there are four main recognition tasks, or slots – bus route names, bus-stop locations, dates, and times. Whereas a conventional system tracks a single value for each slot – i.e., the speech recognizer’s top hypothesis – our statistical system tracks a distribution of many possible values over each slot. Past work in lab studies has showed that this distribution improves robustness to speech recognition errors; but to our surprise, we found the distribution yielded an increase in accuracy for only two of the four slots, and actually decreased accuracy in the other two. In this paper, we identify root causes for these differences in performance, including intrinsic properties of N-best lists, parameter settings, and the quality of statistical models. We synthesize our ﬁndings into a set of guidelines which aim to assist researchers and practitioners employing statistical techniques in future dialog systems.