Applied Statistics Lesson of the Day – Basic Terminology in Experimental Design #2: Controlling for Confounders
January 5, 2014 Leave a comment
A well designed experiment must have good control, which is the reduction of effects from confounding variables. There are several ways to do so:
- Include a control group. This group will receive a neutral treatment or a standard treatment. (This treatment may simply be nothing.) The experimental group will receive the new treatment or treatment of interest. The response in the experimental group will be compared to the response in the control group to assess the effect of the new treatment or treatment of interest. Any effect from confounding variables will affect both the control group and the experimental group equally, so the only difference between the 2 groups should be due to the new treatment or treatment of interest.
- In medical studies with patients as the experimental units, it is common to include a placebo group. Patients in the placebo group get a treatment that is known to have no effect. This accounts for the placebo effect.
- For example, in a drug study, a patient in the placebo group may get a sugar pill.
- In experiments with human or animal subjects, participants and/or the experimenters are often blinded. This means that they do not know which treatment the participant received. This ensures that knowledge of receiving a particular treatment – for either the participant or the experimenters – is not a confounding variable. An experiment that blinds both the participants and the experimenters is called a double-blinded experiment.
- For confounding variables that are difficult or impossible to control for, the experimental units should be assigned to the control group and the experimental group by randomization. This can be done with random number tables, flipping a coin, or random number generators from computers. This ensures that confounding effects affect both the control group and the experimental group roughly equally.
- For example, an experimenter wants to determine if the HPV vaccine will make new students immune to HPV. There will be 2 groups: the control group will not receive the vaccine, and the experimental group will receive the vaccine. If the experimenter can choose students from 2 schools for her study, then the students should be randomly assigned into the 2 groups, so that each group will have roughly the same number of students from each school. This would minimize the confounding effect of the schools.