Related article: where N is the sample size, X is the sample mean, ? is the standard deviation, and the subscripts a and b denote each group. When the black-white difference for a specific test is reported subsequently in the text, this equation has been used to compute it. 34 The Long Term Trend Study with consistent data for the NAEP from the early 1970's through 2004 is now available in mathematics and reading for students tested at ages nine, thirteen, and seventeen. 35 For nine-year-olds, the gap in reading scores expressed as points was smaller in 2004 (26 points) than in 1988 (29 points), but the difference in standard deviations was fractionally larger (.76 standard deviations in 2004 as compared with .74 in 1988). 36 Hedges