Assessing the quality of a security market: a new approach to transaction-cost measurement

I discuss a new method for measuring the deviations betweenactual transaction prices and implicit efficient prices. Theapproach decomposes security transaction prices into random-walkand stationary components. The random-walk component may beidentified with the efficient price. The stationary component,the difference between the efficient price and the actual transactionprice, is termed the pricing error. Its dispersion is a naturalmeasure of market quality. I describe practical strategies forestimating these quantities. For a sample of NYSE stocks, theaverage pricing error standard deviation estimate is roughly0.33 percent of the stock price. If the pricing error is normallydistributed and if it is always a positive cost incurred bythe transaction initiators, the corresponding average transactioncost for these traders is 0.26 percent of the stock price. Thedispersion of the pricing error is also found to be elevatedat the beginning and end of the trading session.