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The Third Variable Problem

The Third Variable Problem


A common error in causal thinking is the overlooking of a third variable relating to both variables involved that better explains cause-effect relationships. A classic example of the third variable problem is the strong positive correlation in any city between the number of churches and the number of bars. While this correlation sounds titillating at first, the explanatory third variable is population size: the more people in any city, the more churches and bars (and stores and parks and schools, etc.) there will be.

The activity

  1. Tell the students that they will be given a headline and some of the "facts" from the article. Ask them if they believe that the data supports the headline?
  2. For each, ask them to consider three to four other variables that may impact/predict/cause the relationship.
  3. How could they reword the article to more accurately reflect the study?

Example Headline: "Diet of fish can prevent teen violence."

  • Participants were a group of 3-year-olds given an "enriched diet, exercise, and cognitive stimulation." They were compared to a control group who did not go through this same program.
  • By age 23 they were 64% less likely than a control group of children not on the program to have criminal records.
  • Assume, of course, that the enriched diet included fish.
  • Note, also, that the article does not mention what the other kids ate or did.

(From Tim Robicheaux, retrieved February 2009 from )

Framing this activity for success

We all want students to carry our teachings into their lives. Often, however, we must make our intentions very plain for students to understand that an assignment will equip them with skills and empower them, and is not an arbitrary hoop through which they must jump.

For this reason, frame each activity using these four steps:

  1. Introduce the activity by making clear the specific critical thinking skill the assignment will give them practice using. They may not understand the precise definition of the term, so provide it in writing on the board or overhead. (As a reminder, definitions are here.)
  2. Share examples of how this skill can serve them in their daily lives (e.g., guiding them to buy better products, improving their performance in other classes, advancing their career, communicating better with people they care about, better understanding their own experiences, etc.).
  3. Conduct the activity as described above, making it as active and interactive as possible. When students can talk about their thinking, that thinking moves forward. Teaching for critical thinking is teaching for active learning.
  4. Conclude the activity by reflecting back to students examples that you saw of them using the critical thinking skill effectively, and reminding them to consider the relevance of that skill in other aspects of their. Repeating the examples given in Step 2 may be appropriate, as students will have a different understanding of the skill once they have experienced the assignment.

Important: Wait Five Seconds!

Among the most useful things we as teachers can do during class discussion is shut our mouths. In an oft-cited meta-analysis of wait-time research, Tobin (1987) described findings of greater engagement and achievement when teachers waited at least three to five seconds after asking a discussion question before speaking again. Above this threshold, higher level thinking was observed and student academic performance improved. However, teachers behaving "normally" only tend to wait about one second. Students need a few moments to digest what they have recently heard, formulate their responses, and work up the courage to speak. Slow down, and more of your students will keep up with you.

Reference: Tobin, K. (1987). The role of wait time in higher cognitive level learning. Review of Educational Research, 57, 1: 69-95.