The correlation between spending and achievement is among the lowest I have ever seen in social-science research: 0.08 on a scale from 0 to 1.N.B.: As a social scientist, I generally consider any correlation smaller than 0.30 to have no practical significance, even if it is statistically significant. The 0.08 correlation they observed would have to be four times as large to achieve practical significance, that is, to explain a non-trivial amount of variation in student outcomes.
Why are outcomes so disappointing? Try thinking of a K-12 public school as an odd type of factory where the raw material is not chosen by the management, public schools teach whoever shows up each fall. The additional spending the article describes is putting more energy, resources, etc., into the "process" by which the raw material is transformed into graduates (or too often drop-outs).
Historically, many of the high achievers in public schools have come from middle and upper middle class homes. As we spend more on education, the quality of the feedstock - the students - is declining. Ever larger proportions of public school students come from subcultures whose attitudes toward education range from indifference to antagonism, at the same time that ever larger proportions come from one-parent homes and/or poverty.
Another factor in declining public school performance is the requirement that public schools "mainstream" any student who can arguably profit thereby. In practice many students of limited ability who in the past would have been kept home or sent to special needs schools are now in regular public school classrooms, using disproportionate amounts of resources and teacher time and pulling down average achievement scores.
Middle and upper middle class parents are having fewer children, on average, than formerly. Dismayed at public school conditions, these parents if financially able are diverting many of their children into private or charter schools. Such schools often do have some control over who they choose to teach. Other potential achievers are being home schooled.
To summarize, public schools now educate more students who for a variety of reasons are not traditional achievers and fewer students who would be expected, a priori, to do well. So we spend more to try to make educated graduates out of youngsters with whom we have traditionally not succeeded very well. We'd like to think that this different "mix" makes no difference, but of course it does.
