A reader writes:

As a working organizer, it surprised me that the principal effect organizers are going for wasn't mentioned in the article or contemplated by the study. The effect we look for when asking a supporter to participate in a visibility campaign is an increase in the commitment of the supporter. Having taken the step of contributing resources – whether money or lawn space – to the campaign, a supporter has a personal investment in the success of the campaign and so is likely to do more. Sometimes this means more donating (and since donations are quantifiable, many campaigns focus on that), but what it will almost always mean is that the supporter will be motivated to advocate for the candidate among friends and family. That advocacy, which comes with commitment, is where the real persuasive value of lawn signs lies.