I'm not sure how meaningful this analysis is. First, as others have noted, just having the word "professor" in the title doesn't mean much: both a full, tenure

I'm not sure how meaningful this analysis is. First, as others have noted, just having the word "professor" in the title doesn't mean much: both a full, tenured professor and an untenured, adjunct professor would have the word "professor." (In my field, mathematics, postdoctoral positions are often called "visiting assistant professors"; these are not tenure-track positions.) Second, again as others noted, salary varies widely across fields, e.g., a med school professor will be paid a lot more than an English professor. Just looking at nominal salary also misses out on important subtleties. For example, med school professors primarily doing research are often expected to pay their own salaries out of grants (since many of them do very little teaching or practice medicine), whereas clinical professors (e.g., those who teach in hospitals) get a salary. The former category is much less stable as it is contingent upon sustained research funding, but the second will not leave time for research (nor would it be expected). Source: I'm tenured at an R1 university in the US.