About Those Private School Achievements

With some frequency I hear that private schools outperform public schools. Generally speaking, those who make the claim present no data. Indeed, there aren't many data that bear on the issue and are free of selection bias. Parents who use private schools tend to be better educated and wealthier than those who send their children to public schools. Wealthier and better-educated parents have children who score higher on tests. In addition, private schools have fights of refusal not available to public institutions. One study, carried out some years ago by the American Federation of Teachers, did find that, when one controls for parental education and affluence, the difference on NAEP math scores between public and private schools disappears. The latest NAEP science results contain an even more intriguing set of numbers. These show the scores that correspond to selected percentiles by school type and by grade. 1996 NAEP SCIENCE RESULTS Percentile 10th 25th 50th 75th 90th Grade 4 Public 103 127 151 172 188 Private 126 145 165 182 197 Grade 8 Public 102 126 151 172 191 Private 123 143 164 182 199 Grade 12 Public 103 126 151 174 192 Private 115 136 156 174 191 These results use the same scale for each grade rather than the usual scale that increases the score with increasing age. Across grades, the differences become smaller as one moves up the grade ladder. The interaction of these two factors is such that, by the time one gets to the 75th percentile of the 12th grade, there is no difference between public and private schools. The differences at the low end of the percentile scale are probably the result of selection factors: private schools taking the students they want; public schools taking whoever walks in the door. The diminishing differences between public and private schools as one moves up the percentile scale are harder to explain. The scores for private schools are similar at all percentile ranks for grades 4 and 8, but lower at all percentile ranks for grade 12. The scores of public school students are virtually identical at all three grades. In the past the lack of differences in 12th-grade scores between public and private schools has been explained in terms of dropout rates. The lowest scorers in public schools are gone, and private schools are said to have greater holding power. This selection bias is held to reduce the differences between the two types of schools in the upper grades. That is a plausible explanation, but it does not explain the discrepancies in these data because of the constancy of the public school scores at all grades and all percentile ranks. What could explain the dip in private school scores? It's not possible to say with any assurance. But recall that the Fifth Bracey Report (October 1995) ended with a survey from Money magazine that concluded that private schools usually weren't worth the money. Said Money, "Here's the bottom line: you are probably wasting your hard-earned money. " One finding in the Money survey was that public schools often had a richer array of advanced course offerings than private schools. Unfortunately, the NAEP mathematics report for the same year does not contain a comparable graph. (It is very frustrating to read NAEP reports because each one contains a number of unique features. It would be desirable to have a standardized reporting format to permit comparisons across subjects.) The science report is NAEP 1996 Science Report Card for the Nation and the States, May 1997. More About Grade Inflation Or Lack of It In last month's Research column, I presented the case that the absence of . 400 hitters in baseball reflects a general increase in excellence, not a decline in batting skills. …