The study looked at stock returns on Mondays compared to other days, using data from as far back as 1928. They found that Monday returns were consistently negative for different types of stocks, including those traded on exchanges and over-the-counter. The results suggest that the pattern is not due to specialists in the market. When looking at individual stocks in the Dow Jones Index, they found that Friday and Monday returns were positively correlated, which goes against common explanations for the phenomenon.