Researchers are increasingly using data from the Nasdaq
market to examine pricing behavior, market design,
and other microstructure phenomena. The validity of
any study that classifies trades as buys or sells
depends on the accuracy of the classification
method. Using a Nasdaq proprietary data set that
identifies trade direction, we examine the validity
of several trade classification algorithms. We find
that the quote rule, the tick rule, and the Lee and
Ready (1991) rule correctly classify 76.4%, 77.66%,
and 81.05% of the trades, respectively. However, all
classification rules have only a very limited
success in classifying trades executed inside the
quotes, introducing a bias in the accuracy of
classifying large trades, trades during high volume
periods, and ECN trades. We also find that extant
algorithms do a mediocre job when used for
calculating effective spreads. For Nasdaq trades, we
propose a new and simple classification algorithm
that improves over extant algorithms.