Requirements of External Validity: Internal Validity & Replication

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  • 0:07 External Validity
  • 1:51 Replication
  • 3:17 Control and Generalizing
  • 5:54 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.

In order to generalize results from studies to the real world, there are a couple of things that are required. In this lesson, we look at the requirements for external validity: replication and internal validity.

External Validity

Solomon Asch was curious about what made people follow the crowd. Why did some people march to the tune of their own drum, while others just went along with whatever everyone else was doing?

To find out, Asch designed a famous experiment on social conformity. He had participants sit in a room with a bunch of other people. They thought the other people were other participants, but in reality they were working for Asch.

Then Asch showed everyone three lines. Two of the lines were similar in length, and the other was either much shorter or much longer than the other two. He asked which lines were similar and went around the room, encouraging everyone to answer out loud.

The people working for Asch all gave the wrong answer. But here's the kicker: when they came to the real participant, many of them gave the wrong answer, too, even though they later said they knew it was the wrong answer!

Asch's study gave insight into what types of circumstances lead to conformity, and psychologists used Asch's study to form theories about why people act the way they do in the real world. But does Asch's study represent the real world? Can picking out which lines are similar in a lab setting really tell us anything about why teenagers succumb to the peer pressure to do drugs, for example?

External validity is the extent to which the results of a study can be generalized to the world at large. Asch's study has been criticized for having low external validity; that is, some people believe that his study can't be generalized to the real world.

Let's look closer at two requirements for external validity: replication and internal validity.


Imagine if Asch did his study and found that people always gave the right answer, despite what others said. 'Aha!' he might think, 'People are not influenced by others.' But then imagine that someone else did the same study and found that people were influenced by the others' answers.

Replication is when a study can be done again and the same general results are found. If a study is highly replicable, that means that it can be done over and over and the same result will be found each time.

Replication is important for external validity because if a study can't be replicated, how can we know that the results are true? If Asch's results are found time and time again, then we can guess that they probably hold true.

And if Asch or other psychologists tweak his experiment (perhaps by using different types of subjects, varying the task, or doing it in a non-lab setting), and then they get the same result, then the external validity of the study is even stronger. After all, if the results hold true for a variety of people and settings, there's a good chance that they can be generalized to the larger population.

In reality, after Asch found that people are vulnerable to social conformity, many other studies replicated his findings. Years and years of studies have found that people tend to conform in a variety of situations.

Control and Generalizing

We know that external validity involves being able to say that the results of the study are applicable to the real world. And we know that Asch has been criticized somewhat because his study doesn't seem to apply to any real-world situations. So why do an experiment like that in the first place? Why not just go out into the real world and do an experiment that involves a real-world experience?

The problem with field research, or research done in the real world, is that there isn't much control. Let's say that Asch wants to see how many students will do drugs when their peers around them are doing drugs. So he goes to a bar and sets up an experiment to test this: he has his confederates (or people working for him) do drugs while hanging out with a participant.

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