One of the interesting anomalies of asset trading in secondary markets is that it is not usually possible to observe actual market clearing prices. Rather, we can only observe transactions as bid and ask price? and infer that the market equilibrium price lies between them. Professor Smidt analyzes continuous transactions data for individual NYSE listed securities during 1977. From it he deduces (1) the behavior of transaction prices, (2) the average size of bid-ask spreads, and (3) the movement of market prices. Given that neither the bid-ask spread nor the actual equilibrium price is observable from the available data, this is an ambitious undertaking. The problem is further complicated by the fact that the transactions data contain three distinct types of trading activity: matching trades at the opening or reopening of the auction market, auction trades (the bulk of activity by number of transactions), and block trades (the second largest activity by dollar volume).