I have seen several stories of late suggesting that minimum wage phase-ins tend to mask the full employment effects of the wage change. That is because people tend to look at employment before and after the wage change itself, when in fact many companies may have already adjusted their employment long before the wage change goes into effect based on the original announcement.
This certainly rings true with me. We decided to close one operation in California after the state passed legislation to raise the state minimum wage (the minimum wage change was one of three factors leading to the closure, the other being the PPACA employer mandate which would be particularly expensive at this location and vexing litigation harassment in this one particular area). This means that for a minimum wage change that does not take effect until July 1, 2014, our decision to reduce staff came in the fall of 2013 and the jobs will go away on December 31, 2013, months before the minimum wage change actually takes effect.
I can certainly see how this would make designing a study to capture the employment effects of the minimum wage change very difficult. From a more cynical point of view, it also makes it far easier for minimum wage supporters to understate the employment effects.
This same phase-in effect can be seen with the Obamacare employer mandate. I criticized Brad Delong for arguing that we would not see any shifts to part time labor until the employment report after the actual start date of the employer mandate. But I know our company had been shifting people to part-time status in anticipation of the start date nearly a year earlier, as had most other retail businesses. While it may be normal for the government to put off working on something until on or after the due date (e.g. the Obamacare web site), private industry tends to start planning and implementation of responses to government regulations months or years in advance.