One measure of academic performance is teacher-given grades.^{16} Although 12 percent of the overall group of students with mental retardation received grades of "mostly As and Bs" in their general education academic classes, and 18 percent received "mostly Bs and Cs," the majority received grades of C or below; 45 percent received "mostly Cs and Ds," and one in four (25 percent) received "mostly Ds and Fs" (table 22). No significant differences were found when comparing parent-reported high-, moderate-, and low-functioning levels of students with mental retardation.
Performance on individual assessments is another measure of achievement. One assessment was attempted for each NLTS2 sample member during the biennial data collection cycle in which he or she was in the 16- through 18-year-old age range.^{17} The NLTS2 direct assessment used research editions of subtests of the Woodcock-Johnson III Tests of Achievement (WJ III) (Woodcock, McGrew, and Mather 2001) that test language arts skills, mathematics abilities, and content knowledge in science and social studies.^{18} NLTS2 also included a functional rating to provide information on youth for whom the direct assessment was reported to be inappropriate. The functional rating was the Scales of Independent Behavior-Revised (SIB-R) (Bruininks et al. 1996), a comprehensive measure of adaptive functioning in school, home, employment, and community settings. To determine the form of assessment for which youth qualified, assessors interviewed the school staff person who was most familiar with a youth and his or her school program; information was sought from parents if youth were no longer in school, including any accommodations that a youth required. Among the overall group of students with mental retardation who participated in the assessments, 79 percent took part in the direct assessments and 21 percent had a functional rating.
Direct assessment results are reported as standard scores, which for the general population of youth in the test norming sample have a mean of 100 and a standard deviation of 15. Thus, in the general population, 50 percent of youth scored at the mean of 100 or above and 50 percent scored below. The performance of the majority of youth in the general population (95 percent) falls within two standard deviations around the mean; that is, scores between 70 and 130. Two percent of youth in the general population receive scores that are more than two standard deviations below the mean—i.e., less than 70—a standard score range classified by the WJ III as being "very low" (Woodcock and Mather 1990). Compared with the 2 percent of youth in the general population who scored in this "very low" range on the subtests, 73 percent of youth with mental retardation scored "very low" on the passage comprehension subtest, as did 65 percent on the mathematics calculation subtest, 59 percent on the synonyms/antonyms subtest, 57 percent on the applied problems subtest, 56 percent on the social studies subtest, and 48 percent on the science subtest (p < .001 for all comparisons, table 23).
Average scores for students with mental retardation on the various subtests of the direct assessment ranged from 56 to 67 (table 24). The average score for passage comprehension (56) was lower than for other subtests, which were 67 in science (p < .001), 65 in social studies and in synonyms/antonyms (p < .001), 63 in applied problems (p < .001), and 61 in mathematics calculation (p < .01). The average scores on the applied problems and mathematics calculation subtests were lower than the science subtest average score (p < .05) and (p < .01, respectively). The average score on mathematics calculation was lower than on social studies and synonyms/antonyms subtests (p < .05 for both comparisons).
Significant differences in the mean standard scores for all six subtests were found across the different levels of parent-reported cognitive functioning. For the passage comprehension subtest, students with high functioning had a mean score of 64, significantly higher than the average score of 53 for moderate-functioning students and the average score of 31 for low-functioning students (p < .001 for both comparisons). Moderate-functioning students with mental retardation also had significantly higher mean standard scores for the passage comprehension subtest compared with low-functioning students (p < .01). For the synonyms/antonyms subtest, students with high functioning had a mean score of 72, compared with the mean score of 62 for moderate-functioning students and 50 for low-functioning students (p < .001 for both comparisons). Moderate-functioning students with mental retardation also had significantly higher mean standard scores for the synonyms/ antonyms subtest than low-functioning students (p < .01). For the mathematics calculation subtest, students with high cognitive functioning had a mean score of 72, significantly higher than the score of 55 for moderate-functioning students and the score of 46 for low-functioning students (p < .001 for both comparisons). Students with high functioning had a mean score of 74 on the applied problems subtest, a significantly higher score than the 58 for moderate-functioning students or 45 for low-functioning students (p < .001 for both comparisons). Moderate-functioning students with mental retardation also had a significantly higher mean standard score for the applied problems subtest than low-functioning students (p < .05). For the social studies subtest, the mean standard score of 70 for students with high functioning was significantly higher than the 62 for moderate-functioning students (p < .001) or 57 for low-functioning students (p < .01). Finally, students with high functioning had a mean score of 73 on the science subtest, significantly higher than the mean standard scores of 64 and 58 for moderate-functioning and low-functioning students, respectively (p < .001 for both comparisons).
If a youth did not meet the requirements for the direct assessment, even with accommodations, he or she was eligible for the functional rating, and a rating form was completed by a teacher if a youth was in school or by a parent if he or she was not. The four clusters of the functional rating scale measured motor skills, social interaction and communication skills, personal living skills, and community living skills. These four clusters also were combined into an overall scale referred to as "broad independence." Functional rating results are reported as standard scores, which for the general population of youth have a mean of 100 and a standard deviation of 15.
In the general population, the distribution of test scores on each cluster is equally divided above and below the mean (Bruininks et al. 1996). Therefore, as seen in table 25, 50 percent of youth in the general population scored above the mean, and 50 percent of youth in the general population scored below the mean. Of those students who scored below the mean, 2 percent scored more than 2 standard deviations below the mean on the measure of "broad independence." In contrast, 89 percent of the overall group of youth with mental retardation who received the functional rating were reported to have scores more than two standard deviations below the mean on "broad independence," with a mean standard score of 23. In addition, more than three-quarters of students were reported to have scores more than two standard deviations below the mean on each of the four functional behavior measures. The mean standard scores on each of the scale's clusters for students with mental retardation who received the functional rating are found in table 26.
Among students with parent-reported moderate and low functioning,^{19} significant differences were found in the mean standard scores for the four clusters of the functional rating scale. For broad independence, moderate-functioning students had a mean standard score of 30, significantly higher than the mean standard score of 11 for low-functioning students with mental retardation (p < .01). Both moderate- and low-functioning students scored highest on the "personal living" cluster, yet moderate-functioning students' mean score of 55 was significantly higher than low-functioning students' mean score of 27 (p < .001). Moderate-functioning students with mental retardation also scored significantly higher on the "motor skills" cluster (53 percent vs. 25 percent, p < .001) and the "social interaction and communication" cluster (38 percent vs. 16 percent, p < .001). Both moderate and low-functioning students scored highest on the "community living" cluster. Again, the moderate-functioning students' mean score was significantly higher than the low-functioning students' mean score (26 percent vs. 9 percent, p < .01).
^{16} Grades reported by teachers in the NLTS2 Wave 1 (2002) general education teacher survey.
^{17} Assessments were conducted in 2002 and 2004.
^{18} See Wagner et al. (2006) for more information about the direct assessment subtests and the data collection process.
^{19} Items with fewer than 30 respondents are not reported in this fact sheet. Consequently, high-functioning students with mental retardation were not considered in the analysis of the functional rating scales.