About two years ago, I wrote a blog post about my continuing efforts to teach experimental design in my nonmajors biology class. That post (Little Changes, Big Difference) detailed my use of the “Marshmallow Test” film clip to generate questions and hypotheses about the future adult behavior of the children featured in the video.

I used the same activity last week, and it dawned on me at the end of class that I wasn’t doing a good job teaching a key idea. The exercise asks students to design their own experiments, specifying the independent variable, dependent variable, standardized variables, and control. I learned that many students assume those four items must be mutually exclusive. As a result, they often come up with a combination of “independent variable” and “control” that makes no sense. For example, one student designed an experiment to test whether kids who waited (and “won” a second marshmallow) would have higher grades at age 20 than students who didn’t wait. According to this student, the independent variable was the “waiters” and the control was the “non-waiters.” (A better answer for the independent variable would have been “Behavior in childhood marshmallow test,” and a reasonable control group would have been the children who hadn’t waited.) But since she assumed that the independent variable and control were mutually exclusive, she figured they couldn’t have anything to do with one another.

In the next class (5 days later), I tried to correct the misconception by working through an example. I showed a very simple graph, with “Amount of fertilizer” on the X axis and “Yield (kg)” on the Y axis. The first clicker question pointed to the Y axis and said, “These are possible values for the (a) independent variable (b) dependent variable (c) control and (d) standardized variable.” The second clicker question asked the same question about the X axis. The third clicker question asked whether one of the values on the X axis was the control. The final slide in the series (shown right) reminded students that the zero value was the control.

Next time I teach this activity, I will be aware of the independent variable-control confusion and ask these questions as part of the experimental design activity itself (instead of waiting until the next class period). Then, I will find a way to test whether students can apply this concept to their own experiments. I predict that this series of questions will boost performance overall.

In part 2 of this post, I will describe a totally different approach to teaching experimental design in very large classes: by letting students make the predictions, be the subjects, report their data, and draw their conclusions, all without leaving their seats.