Politicians seem to have temporary set aside the debate about improving our schools, but you can bet that when the issue rises again, one solution will be raised, over and over: Improving student/teacher ratios--that is, hiring more teachers. But is it really a silver bullet for increasing results? What sort of results can we expect?
The graph above offers a few clues--but unraveling them takes a bit of explanation. The crucial point being: Adding teachers might improve student performance relative to past results, but it's a weak lever for effecting aggregate improvements.
So, let's dig into the graph. Each of the lines--colored in blue or green--represents data from a single state. To the left is that state's student/teacher ratio; to the right is that state's average SAT score.
The graph looks sort of confusing at first, but it actually does a pretty good job at showing that student/teacher ratios and SAT scores aren't closely related. If they were highly correlated, you'd expect to see lines with slopes all at a 45-degree angle (whether sloping up or down). But as you can see, they're actually a tangle. The states with the highest SAT achievement have relatively low student/teacher ratios--but those ratios alone don't account for their performance, since plenty of other states have similar ratios but don't score nearly as well.
The confounding factor, I expect, is that there are several other variables much more closely associated with SAT scores--the chief one being family income .
And the point of the graph would seem to be: Adding teachers might improve performance at the school level--but really raising educational achievement remains a hard problem, with no easy (or easily proven) solutions.
The real question is: Can you recreate the advantages of living in a wealthier home, inside a classroom? One reason to believe we can: The Harlem  Children's  Zone , a program that began  with the simple idea of teaching the child-raising mores taken for granted in high-income households to the less the fortunate. Which sounds creepily paternalistic, but data proves there's a difference in how poor and rich raise their kids: For example one famous study  found that high-income parents, compared to low-income parents, speak more than three times the number of words to their preschoolers. Those development differences follow a child throughout their life. So add teachers. But add them early, and train them better.
[Graph by Flowing Data ]