Gummi Bear Design of Experiments: Choosing Factors Not to Study

Cody Steele 15 February, 2012

Of the 24 variables from the fishbone diagram that I made up earlier, I picked 5 last time to intentionally manipulate as factors in my study. That means that there are 19 other variables (plus ones that you thought of on your own) that we still have to consider to do design of experiments. What should we do about these variables? It depends.

We’ll talk about the easiest strategy in design of experiments this week, using the orientation of the bear on the popsicle stick as an example.

Leaving a Variable the Same

Leaving orientation the same while we change the other factors guarantees that orientation won’t affect how far the bear goes. It guarantees that orientation doesn’t interfere with the factors that you really want to study, which is an important goal of design of experiments.

Leaving orientation the same can be particularly desirable when you already know that there’s a best way to orient the bear. If you’ve tried, you’ve probably noticed that it’s hard to get a gummi bear to stand on its head on a popsicle stick catapult without falling off. The most stable orientation is to have the gummi bear on its back. So we might choose to leave a variable the same because we already know what we want to do with the variable. I will, in fact, use this strategy for several of the variables on the fishbone diagram: type of candy, type of rubber band, type of popsicle stick, surface where I’m launching, and measuring device.

Problems that Could Arise with Not Manipulating a Variable

When we leave a variable the same, we also guarantee that we won’t learn anything about it. We’ll never know if facing a gummi bear with its feet towards the rubber band or its head towards the rubber band makes any difference in how far the gummi bear goes. If you have a number of variables that you suspect won’t matter, but you want to check whether you need to do more experimentation, you can use those variables to define blocks. That’s what I’ll discuss in my next post.

Want to get a preview? Here’s 3 minutes and 21 seconds about blocking to whet your appetite.