Variables and Confounds

A confounding variable is something that varies systematically with the levels of the independent variable. It is not just a general problem with the study, but a very specific type of problem. A confound reduces the internal validity of an experiment.

Another problem that a study might encounter concerns external validity. Are the findings of the study generalizable and applicable to other groups or other settings? In general, we can reasonably expect our results to apply to other people/situations that are similar to those we studied, but we must be cautious about generalizing beyond that.

A random variable is something that varies randomly within and across the levels of the independent variable. For example, there are often individual differences among participants in each condition. Random variables can make it more difficult to find a statistically significant difference between the experimental conditions, but they are not confounds and do not present problems for the internal validity of a study.

Below is a description of a study followed by a list of 9 potential issues. For each one, determine whether it is a confound, a random variable, or an external validity issue. Explain why you think so.

A researcher studied whether it is helpful to include pictures on medicine instructions to encourage patients to take their medicine correctly. Instruction A contained ‘standard’ verbal instructions that are currently used, while Instruction B included both verbal instructions and pictures (such as a picture of a clock or calendar to tell people how often to take the medication). The researcher obtained permission from the doctors at a local hospital to administer two types of instructions to 100 of the hospital’s patients. 50 of the patients were randomly assigned to be given Instruction A and the other 50 were given Instruction B. All patients were on medications that involved taking 1 pill twice per day. Each day the patients filled out a form indicating when they took their medications (i.e. whether they correctly followed instructions).

1. Instruction A contained more complex words in the verbal instructions than Instruction B. 2. Due to their different medical conditions, some patients had to take different drugs than others. 3. All the patients were from one hospital in one specific town. 4. Some of the patients might not take their medication to avoid side effects. 5. The medications were put into larger bottles for Instruction B so that the pictures could fit on the label. 6. Patients were of different ages, and older people might have a harder time understanding the instructions. 7. Instruction A was written in smaller print than Instruction B. 8. The patients self-reported whether or not they had taken their medication properly, and some of them may have not reported honestly. 9. All the patients in the study were on medications that had very simple instructions: “Take 1 pill twice per day.” This is much simpler than the medication requirements of many other patients that typically involve multiple different medications, some taken on a daily schedule, and others taken weekly or monthly.