THE EXPERIMENT: A Tool for Allowing Us to Determine Whether "A" Causes Changes in "B."

The experiment is one of several research methods. It is valuable source of information because it is the only method that allows us to make cause-effect (causal) statements: statements about what makes something happen; statements about what factor influences another; statements like "That behavior occurs because...." or "The effect of noise is ....").

Definition of the simple experiment: a research tool that allows scientists to find out whether a treatment influences (causes) a given behavior or mental characteristic by randomly assigning some participants to get the treatment and other participants to not receive the treatment.

Brief overview of the Simple Experiment

No treatment group Average Score
Treatment group Average Score

No caffeine group 15 seconds
Caffeine group 9 seconds
Average Time to Run Maze (Skinner, 1938)

Take a quick quiz

Why do we do experiments?

To make cause-effect statements

Two types of hypotheses tested in experiments:

1. Experimental hypothesis: prediction that the

treatment causes an effect. It can be proven wrong.

2. Null hypothesis: prediction that the treatment does not cause an effect. It also can be proven wrong. It cannot, however, be accepted.

Joke illustrating problem of accepting the null hypothesis.

More serious problem that could result from accepting the null hypothesis.

What happens if we disprove the null hypothesis?

(Hint: If the statement "I did not eat the candy bar" is false, what did I do?)

Why do we have two groups?

(Hint: In the Skinner experiment with the rats running the mazes, what could we have concluded if we had only used a caffeine group. That is, what could we have concluded about the effects of caffeine if all we knew was that the rats getting caffeine ran the maze in 9 seconds?)

How can we avoid comparing apples with oranges?

(How do we know that our treatment group and control group were similar before we introduced the treatment?)

Random assignment to treatment involves using a

system where everyone who participates in the study has

an equal chance of being put into the treatment group.

Time to take an ungraded quiz.

What's the problem with random assignment?

Hint: Suppose we get these results:

Experimental Group = 75%

Control Group = 74%

Could these results be due to random assignment creating groups that were slightly different before we introduced the treatment?

How can this problem be solved?

Tests of statistical significance determine if the difference is to big to be due to chance alone

The tests look at two factors:

1. They look at the size of the difference.
The bigger the difference between the groups, the more likely the results are to be statistically significant. For example, if the Experimental group averages 95% and the control group averages 45% on our test, that difference would probably be statistically significant. (Intuitively, you do the same thing. If your team gets beat by one point, you point out that the other team was lucky. You don't have to concede that the other team is better. However, if they beat your team by 30 points, you may have to admit that the other team is better).

2. They look at the number of participants.
The more participants that are used, the more likely the results are to be statistically significant. (Why? Because if you only have a few participants, the groups might be very different at the beginning of the study. However, if you have 100 participants in each group, the groups should be pretty similar before the start of the study. If they are very similar at the start, then, if they are even slightly different at the end, that difference could be due to the treatment. Similarly, in sports, if one team beats another in a seven game series that's more convincing evidence of the team's superiority than winning a single game.)

Two possible verdicts from statistical tests

1. statistically significant:

you are sure beyond a reasonable doubt (your doubt is less than 5%) that the difference between your groups is too big to be due to chance alone.

So, if the difference between the treatment group and the no-treatment group is too big to be due to chance alone, then some of that difference is probably due to treatment. In other words, the treatment probably had an effect.

2. not statistically significant:

you are not sure, beyond a reasonable doubt, that the difference between the groups is due to anything more than just chance.

So, you can't conclude anything. The results are inconclusive.

Time to take a time out to test your understanding of statistical significance.

Conclusions about the Experiment

The goal of the simple experiment:

is to find the causes of behavior. That is, the goal is to find rules that will allow us to understand and control behavior.

Why the simple experiment can accomplish that goal:

it uses random assignment. Random assignment accomplishes two things:

Implications of the fact that the experiment depends on random assignment:

Take another ungradedquiz.

Review of the Simple Experiment

By now, you should be able to:

  1. Explain why there are certain research questions that cannot be answered by using an experiment.

  2. Explain why the case study illustrates the value of having a control group.

  3. Explain why the null hypothesis can't be proven right.

  4. Explain why researchers use double-blind techniques.

  5. Explain why only experiments can answer causal questions.

  6. Explain why a good hypothesis is one that could be disproven.

  7. Explain the difference between an independent variable and a dependent variable.

  8. State a rule for knowing whether a study that uses two groups is an experiment. State two implications of that rule.

  9. Explain why the following statement is false: "To find out if the treatment had an effect, simply see if the treatment group scored differently than the no-treatment group."

  10. Explain what it means if the results are statistically significant.

  11. Explain what you can conclude if the results are not statistically significant.

  12. Explain why the simple experiment is better than the before-after study.

  13. Explain the difference between random assignment and random sampling.

  14. Explain why you can't scientifically study the effects of race, gender, or personality.

  15. Take this quiz until you can easily get 100% on it.
  16. One last quiz on experiments

On to using your knowledge of research methods to be a better critical thinker
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