Most of my students came in on Saturday to give up a few hours of their time to continue calling people for our survey. We got 57, which isn't our best day, but puts us up to 231 responses, which puts us only 35 short of our minimum of 266. I'd rather get 300, to make up for a couple of respondents who didn't finish the survey, and to make sure we have enough coverage on each question, since "No Response" crops up occasionally on a few questions. So just one more session of calling, or 1 and 1/2 sessions, should get us there. Importantly, we've done well enough that the students are still reasonably enthusiastic.
I also told them if we quit now, we've done all this work for nothing, but if we do just a bit more, we'll have been successful. I felt a bit guilty as I said it, because none of them have taken my political economy class, so none of them knew I was using sunk costs as reason to keep putting in more effort. Maybe next year one of them will take that class and call me on it! But sunk costs arguments seem so irresistible to humans that they're a very useful motivational tool.