[whimsy]

Well, the primary goal is, of course, to raise the income of minimum-wage earners. Failure criteria probably include things like a) a marked increase in businesses that rely on minimum-wage earners going out of business relative to other and/or b) a marked increase in unemployment among minimum-wage earners. I don't know what thresholds are reasonable. (Possibly confounding: as minimum wage increases, the available labor pool will also increase.)

In the case of Seattle, they commissioned two pieces of research for this ordinance:

UW Evans School of Public Affairs study: Local Minimum Wage Laws: Impacts on Workers, Families and Businesses [1] University of California, Berkeley study: Who Would be Affected by an Increase in Seattle’s Minimum Wage? [2]

The second paper addresses a lot of the questions I suspect you're interested, and it addresses them with empirical evidence based on previous such experiments in other cities.

[1] http://murray.seattle.gov/wp-content/uploads/2014/03/Evans-r... [2] http://murray.seattle.gov/wp-content/uploads/2014/03/UC-Berk...