Some large employers with multiple locations in the state of Washington, such as retail or restaurant chains with company-owned stores, file a single quarterly report to cover employees at all locations. This quarterly report may list a single location, such as a corporate headquarters, in the address field and not provide any method of ascertaining whether a specific employee worked at a Seattle location.Chains with company-owned stores are significant minimum-wage employers.
And then we have:
In our baseline analysis we focus on single-location establishments for which we can determine with certainty if they are subject to the ordinance.In other words, their focus was on the corner shop, rather than big chains. But these are some of the least stable businesses and subject to every change in the economic wind. In particular, they're very subject to rises in rent, which are a big deal in Seattle, and the researchers seem to have inadequately controled for this.
The study is an interesting one, but the popular use of it, at least, overclaims hugely. Since the study contradicts most previous work in the field, I think it deserves to be treated as an interesting study, rather than new authoritative work.