My Tip Jar Experiment

One of the two classes that I’m taking this semester is in design of experiments, which is more fun that it might sound.  I’ve done these before, and I think that the one that I’m doing now is pretty cool.

I ran an OFAT1 experiment back in middle school using paper airplanes and a contraption that ensured that they’d be propelled with the same force each time. Dad and I built a wooden box, entrapped a rubber band inside of a staple on the front of the box, and placed a clothespin at the back of the box that would hold the tail of the plane such that the nose of the plane a standard length from the front edge of the box.  Opening the clothespin launches the plane.  I ran five trials for each plane, averaged them, found the variance, and tabulated my standings.  I didn’t get first place,2 but I did have fun.

This experiment is more interesting and advanced.  I’m running a fractional factorial experiment, which means that I can test four factors in just eight runs without losing anything but higher-order interactions that aren’t likely to matter very much.  I’m using the coffeeshop that I spend a lot of time in, largely because I have the trust of the baristas that I’m not going to screw them over.  I have four factors:

  1. Time of day of the shift.  There are two shifts each day.
  2. The size of the tip jar.
  3. The opacity of the tip jar.  These two factors require that we have four tip jars.
  4. Whether the tip jar is seeded or not.

My premise is this: tipping baristas is a social phenomenon.  “Do I tip her?  All she did was make me a latté,” is a valid question.  Anything that we can do to shake up the social norm and show that, yes, people tip baristas is a good thing.  But I have no idea what factors will work.  That’s why you experiment, people.

I’m sitting here waiting on the end of the third shift under experiment.  #4 and #5 happen tomorrow, and the rest will conclude by Thursday.  I’m having fun, and so are the baristas, especially since I’m giving them money to participate.  I got some very interesting results yesterday, and I’m getting some predictable ones today.  Data!  I want more data!

I’ll get to do this one more time, either as a full 2^3 factorial that would test every factor combination, or by doing the alternate fraction of this 2^(4-1), which would test the other eight combinations.  I’d get to do the former if I have an obvious factor that has no value in the analysis of the first experimental run; if I don’t have conclusive data, I’ll do the latter.  I could also run another fractional factorial by dropping a factor and adding another one.

The ladies already want me to run more experiments.  I guess that I know what I’m going to be doing on my own time this summer.


  1. One Factor at A Time. In this case, it was the plane itself that changed. 

  2. Assholes!