1. Understanding Experiments
Lecture 13
Secs. 3.5
Fri, Feb 6, 2004

2. Understanding Experiments
Recall that in an experiment, the researchers may control the explanatory variables.
See Example 3.7, p. 145.
What are the explanatory variables?
How are they controlled?
What are the treatments?

3. Let’s Do It!
Let’s do it! 3.5 – Component Lifetime.

4. Designing an Experiment
To test the effectiveness of a treatment, we need something to compare it to.
Treatment group – A group that receives one of the actual treatments.
Control group – A group that receives no treatment, but is otherwise identical to the treatment groups.

5. Example See the example at the bottom of page 149.
What is wrong with the experiment?

6. Random Allocation
Continuing the previous example, suppose the researchers decide to divide 100 subjects into two groups.
Group 1 receives Vitamin C.
Group 2 does not receive Vitamin C.
How should they be divided?

7. Random Allocation
The assignment to the groups should be random (a randomized design).
The subjects could be numbered 1 – 100.
Then use a random number generator to obtain 50 (distinct) random numbers from 1 – 100.
Those 50 subjects would be assigned to one group.

8. Response Bias
Are the subjects in the control group aware that they are not receiving Vitamin C?
Are they aware of the purpose of the experiment?
If so, then they may be more likely to report cold symptoms.
Similarly, the treatment group may be less likely.

9. Response Bias
Response bias – The tendency of the subject to show the response that he believes the researcher is expecting.

10. Placebos
To help eliminate response bias, the researchers could give a similar tablet, but containing no Vitamin C, to the subjects in the control group.
Such a tablet is called a placebo.

11. Blinded Experiments
To reduce or eliminate the response bias, we can use a blinded experiment.
Single-blind experiment – The subjects do not know which group they are in.
Double-blind experiment – Neither the subjects nor the researchers know which group is the control group.

12. Placebo Effect
There is still a possible problem, even in a double-blind experiment.
The subjects in the control group may respond to the fact that they are receiving attention and a tablet, even though the tablet itself has no effect.
This is called the placebo effect.

13. Let’s Do It!
Let’s do it! 3.7, p. 155 – Study Suggests Light to Back of Knees Alters Master Biological Clock.
Was there a control group?
Was this a blinded study?
Could there be a placebo effect?

14. Let’s Do It!
Let’s do it! 3.8, p. 156 – Controlling for Another Factor.
Discuss the flaw(s) in this experiment.

15. Assignment Page 148: Exercises 12 – 16. Page 157: Exercises 19 – 23.