My Tipping Jar Experiment, Round 2

So the first round of the tipping jar experiment went well.  There were two dominant factors: time of day and size of the jar.  I was not surprised that morning tipping was better than evening tipping, as the store is busier early than late.  I was surprised by the degree to which it dominated the results.  You use fractional factorial experiments to screen for results.  Normally, I would reduce this to a 23 experiment1 by dropping an insignificant factor; instead, I have changed how I collect my data and am re-running with the other three factors.  Instead of morning and night shifts allowing me to collect the data in 5-6 days, I’m using each full day as an experimental run, which will take me eight days.

I fully expect that tip jar size — larger was better, which surprised me — will continue to be a key player, but I want to know if opacity and seeding have main effects, and I want to know if any interactions occur.2  With a full factorial — even with a single replicate — I’ll be able to create a good reduced model once I see which main effects and interactions have any meaning.  It may be that tip jar size is the only factor that matters, but I won’t know until I take data.

I start in the morning and finish next Friday, which gives me 11 days to pull the data and plots together to write a paper.  It’s going to be a furious finish to the semester.  The big thing is that I now have to be there at least some of the time every single day for the next week-ish.  I’m there most every day, but now I have to make a concerted effort.

For future work, we’re going to refine the testing a bit.  Weekdays and weekends have different clienteles.  My tentative plan is to take data M-Th and F-Su, using those as blocks.  That really slows down my time to get results, but I won’t be on a schedule.  The baristas seem really interested in the results of this, which probably doesn’t surprise you.  They have ideas, too, and I’m the man that knows how to make the data happen.  It may take us all summer, but I bet we’ll be getting a good result at the end of it.  I’m already making plans!


  1. Three factors, each with two possible states. 

  2. E.g., how jar size and seeding interact