Requirements of External Validity: Internal Validity & Replication

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.

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