Because of recent changes to the science assessment, results from 2009 cannot be compared to those from previous assessment years. According to the National Center for Education Statistics:
The content of the NAEP science assessment is guided by the 2009 NAEP science framework. It provides the theoretical basis for the assessment and describes the types of questions that should be included and how they should be designed and scored. As with all NAEP frameworks, the science framework was developed under the guidance of the National Assessment Governing Board with input from hundreds of individuals across the United States, including some of the nation’s leading scientists, science educators, policymakers, and assessment experts.
The 2009 framework replaces the one used for the 1996, 2000, and 2005 science assessments. A variety of factors made it necessary to create a new framework: the publication of National Science Education Standards1 and Benchmarks for Scientific Literacy2, advances in both science and cognitive research, the growth in national and international science assessments, advances in innovative assessment approaches, and the need to incorporate accommodations so that the widest possible range of students can be fairly assessed.
The assessment resulting from the 2009 framework will start a new NAEP science trend line so results from 2009 cannot be compared with results of previous science assessments. Whenever changes are made to a framework, efforts are made to maintain the trend lines that permit the reporting of changes in student achievement over time. If, however, the nature of the changes made to an assessment are such that the results would not be comparable to earlier assessments, a new trend line is started.
For a more detailed explanation and a comparison of the two frameworks see: http://nces.ed.gov/nationsreportcard/science/whatmeasure.asp