What's the Independent Variable?

Laura Waters makes an interesting juxtaposition:

Here's dueling perception of NJ high school students' performance on Advanced Placement exams. From the Record and Trevor Packer, vice president of the AP program, the article entitled "NJ Students Score Resounding Success":

Members of New Jersey's Class of 2010 who took Advanced Placement tests can be proud: 72 percent of their exams had scores of 3 or above, the highest passing rate in the nation.

From the Governor's Office, a press release entitled "NJ Students Most Prepared for College, Yet Achievement Gap Remains for Minorities": 

Wealthier New Jersey students are among the best prepared in the nation to succeed in college, according to the College Board’s 7th Annual Report to the Nation released today. But low-income and minority students don’t take as many advanced-level courses and are not as ready for higher education, further demonstrating the need to implement Governor Christie’s education reform agenda.

Back in grad school, we learned that the "independent variable" was the thing that changed, and the "dependent variable" was the result: smoking is the independent variable, lung cancer the dependent variable.

What's the independent variable here? What is the difference between "wealthier" (notice it's not "wealthy") students and low-income students? And how does Christie propose to address this independent variable?

Well, here's his new, vaunted "reform" website.  The only specific policy it mentions:

Teaching can no longer be the only profession where there are no rewards for excellence and not consequences for failure. Governor Christie is committed to eliminating teacher tenure now.

Except that tenure obviously isn't the independent variable - "wealthier" districts have tenure.

Is it "choice"? I don't see how: leaving aside the elite private schools that cater to the very wealthy and/or very talented, most private schools are in the cities. And there are many choices for high school students in a district like Newark; there aren't nearly as many for kids in small, "wealthier" districts.

Is it charter schools? Couldn't be: charters are almost all in the poorer urban districts. Charters are practically unheard of in the "wealthier" districts.

So the things Christie proposes to "fix" the "failing" schools - the ones that aren't as "wealthy" as the VERY successful ones - don't address the independent variable. Gosh, what could the difference be between a district full of low-income students and a "wealthier" district? What could possibly affect student learning that differs between these two types of districts? What's the independent variable?

Thinking...