I think the argument that Forbes systematically favors LACs over STEM-heavy research universities by using 4-year graduation rates is a red herring. Many LACs do boast high 4-year grad rates, e.g., Williams 88.1%, Amherst 87.0%, and Swarthmore 87.1%. But so do some STEM-heavy research universities like Princeton (25.5% engineering majors, 89.7% 4-year grad rate) and Johns Hopkins (19.4% engineering majors, 87.7% 4-year grad rate).
And some LACs are pretty STEM-heavy in their own right, e.g., Pomona (36% of degrees conferred in STEM majors), Swarthmore (34%), Haverford (34%),and Amherst (33%), as compared to 38% at JHU and 42% at Princeton.
Moreover, the schools at the top of the Forbes ranking don’t necessarily have the highest graduation rates. Caltech’s 84.7% 4-year grad rate is similar to Harvard (85.8%) and higher than Brown (82.9%), MIT (81.6%), and Stanford (75.2%) which all rank well above Caltech in the Forbes ranking.
Some STEM-heavy schools like CMU (72.1%) and Case Western (63.6%) do have lower graduation rates, but CMU’s rate is in the same ballpark as UC Berkeley (73.2%), UCLA (73.6%) and Michigan (74.7) which outrank CMU on the Forbes list. And finally I’d note that neither CMU nor Case Western meets 100% of need, which to my mind seems a likelier explanation why a higher percentage of their students are unable to graduate in four years. The weaker financial aid systems of schools like CMU and Case Western also contribute to their students carrying a higher debt load at graduation, a factor that weighs more heavily in the Forbes ranking than graduation rates.