What's your MBA really worth? Well, a lot depends on what you mean. In their marketing materials, elite business schools focus on starting salaries, which can be quite impressive. But what about 5, 10, even 20 years out? For a long time, that's been terra incognita—a no-man's-land of guesswork and supposition. Not anymore.

New research commissioned by BusinessWeek suggests that when it comes to the post-MBA earnings accrued by graduates of top business schools over the span of their careers, not all schools are created equal. Some schools that start out strong with six-figure salaries sometimes sputter and stall, leaving grads with less-than-impressive salaries after 20 years in the workforce. And some schools where grads earn modest salaries out of the gate end up with the strongest of finishes, in some cases doubling their cash compensation after 20 years and overtaking better-ranked rivals. (See our interactive table MBA Pay Through the Years at 45 Top Programs.)

The research comes from PayScale, which collects salary data from individuals through online pay comparison tools. BusinessWeek asked PayScale to dig into its database of 80,000 graduates of 45 top MBA programs and calculate their median cash compensation—salaries and bonuses—during the first five years of their careers and after they have an average of 5, 10, 15, and 20 years under their belts. BusinessWeek then used that data to calculate an estimate of median cash earnings over the entire 20-year span.

The data suffer from some inherent limitations—it doesn't include stock or options, and the pay data for some smaller schools at the 20-year mark may be based on fewer than 100 pay reports. It also does not track the same graduates over time; it reflects the experience of individuals who graduated at various points throughout the last 20 years.

Read the full article here.