A study compared firms with and without an audit committee. For samples of firms of each type, the extent of directors' ownership was measured using the number of shares owned by the board as a proportion of the total number of shares issued. In the sample, directors' ownership was, overall, higher for firms without an audit committee. To test for statistical significance, the Mann-Whitney U statistic was calculated. It follows that (U - µU) /σU was found to be 2.12. What can we conclude from this result?