Stock return anomalies represent an interesting challenge to financial theorists and practitioners alike. Financial theory suggests that profit opportunities in the market are associated with bearing economically important risk, but these anomalies seem to be profit opportunities without any good economic justification.

The challenge for practitioners is to decide whether the anomalies will persist in the future, in which case it might be profitable to trade on them. It is possible that the anomalies are just coincidence and a testimony to how many different patterns people have tested for. It is also possible that they were profit opportunities in the past, but that competition will eliminate them now that they have been recognized.

One common statement about anomalies is that they are ``within the spread'', which is to say that trading on them would not be profitable enough to overcome transaction costs including the bid-ask spread. This assertion may be true, but misleading. For example, we can profit from an average decline over the weekend by delaying any purchase planned on Friday until Monday. This strategy incurs the same cost as buying on Friday.