Sample sizes and rounding can result in apparent inconsistencies. For example, in the table above for both 1996 and 2005, the percentage of fourth-grade students performing at Advanced nationwide is 3, yet the numbers are marked as being significantly different. The actual unrounded numbers are 3.27 in 1996 and 2.54 in 2005, a 0.73 percentage point difference that is statistically significant.
View more information about science achievement levels on the NAEP website. Read the specific definitions of the Basic, Proficient, and Advanced achievement levels for grades 4, 8, and 12.