Researcher Variables that Affect Internal Validity

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  • 0:05 Internal Validity
  • 1:43 Researcher Bias
  • 3:27 Subject Selection
  • 4:48 Researcher Personality
  • 5:55 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

Scientists want only the independent variable to affect the outcome of their studies, but sometimes it is the things they do themselves that change the outcome. We'll look at three common researcher-related variables: researcher bias, selection bias, and researcher personality in this lesson.

Internal Validity

Joan is an educational psychologist. She's doing research on what teaching technique works best for college students. She has three different techniques, which she calls A, B, and C. She also has three different college professors teaching an introductory psychology class. Each professor will try out a different technique, and at the end of the semester, the students who do best on the final will tell Joan which technique is the most effective one.

Sounds pretty simple and straightforward, right? Joan has an independent variable (teaching technique), and she believes it will affect her dependent variable (student performance on the final). But what if one of the professors has students who study more often than the students in the other professors' classes? What if one of the professors ends up sick and unable to teach the second half of the semester?

Internal validity is the extent to which a researcher can say that only their independent variable caused changes in the dependent variable. Other variables that might influence the dependent variable (like students' study time or professor illness) are called extraneous variables. The goal of any study is to have high internal validity by having no extraneous variables. Of course, in the real world this is not always possible, but there are some things that researchers can do to control or eliminate the effects of certain extraneous variables. Let's look at extraneous variables that have to do with the researcher and how to control them.

Researcher Bias

Joan believes that teaching technique A is better than teaching techniques B and C. She thinks that the students in the class with the professor using technique A will do better on the final exam and prove her theory that technique A is better than the others. Joan's belief that technique A is best is a hypothesis, but it can also be a source of researcher bias.

In researcher bias, the researcher's beliefs influence the outcome of an experiment. For example, what if Joan grades the final exams for the technique A class in an easier light than those in the other classes? In this case, Joan is influencing the results; the students in the class exposed to technique A might have learned the material better or they might just have scored better because of Joan's grading.

The thing about researcher bias is that it is often unknown even to the researcher. Maybe Joan doesn't realize that she's grading the technique A class finals easier. She's looking for evidence to support her hypothesis, and she sees it and goes with it, not realizing that she's displaying bias. Or maybe Joan grades the finals for technique A last and by that time she's tired and goes through them quickly, not noticing as many errors as in the ones she graded earlier. Again, this could be a cause of bias.

There is always bias; if there wasn't, we would never have hypotheses! But to control for the impact of bias on the results, Joan could have someone else (or two other people) score the finals and not tell them which finals are from which group. That would help keep her bias from impacting the results.

Subject Selection

A specific type of researcher bias that pops up sometimes is selection bias. In this case, the bias appears at the beginning of the experiment, when selecting and assigning subjects to conditions. For example, let's say that one of the three professors is a really good, really experienced professor. Students who are psychology majors sign up for his section of the introductory class, while the non-psych majors are left with the other two less experienced professors.

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