Ten years ago, a psychologist specializing in male-female relationships authored a book detailing the way in which he believed couples could improve their relationships. His publisher recently was interested in the effect the book has had on married men. The publisher interviewed 32 married men and asked each to rate his marriage, on a scale of 1 to 20, both before reading the book and after reading the book. For each man, the difference between the two ratings was recorded. These differences are summarized in the histogram below. Note that a negative difference means that the marriage rating before reading the book was higher than the marriage rating after. Based on this histogram, estimate the standard deviation of the sample of 32 differences