Understanding Experiments

Lecture 12

Section 3.5

Tue, Feb 6, 2007

Example

Suppose a drug is given to 100 patients suffering from a particular disease.

After 2 weeks, 90% of the patients have recovered.

The researchers conclude that the drug was effective.

What is wrong with this?

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 Group

1. The rest would be assigned to Group 2.

Treatment and Control Groups

Treatment group. Control group.

Random vs. Nonrandom Allocation

Would it be wrong to allow the individuals choose whether to be in the treatment group or the control group?

Would it be wrong for the researchers to decide, subject by subject, who goes in which group?

Why?

Response Bias

Are the subjects in the treatment group aware of the purpose of the experiment? If so, will they be more likely to report feeling well?

Are the subjects in the control group aware that they are not receiving the drug? If so, will they be more likely to report feeling ill?

What can we do about this?

Placebos

Placebo.

Placebos

Placebos

Everybody in the treatment group gets is administered the drug.

Everybody in the control group gets the placebo.

The researchers look for differences in the groups’ recovery rates.

Blinded Experiments

Of course, the subjects should not know whether they are receiving the placebo or the genuine treatment.

Single-blind experiment

Experimenter Bias

Experimenter bias This is similar to response bias except that

it is caused by the experimenter, not the subject.

How can we control for experimenter bias?

Double-Blind Experiments

Double-blind experiment

The Placebo Effect

The placebo effect is when a subject responds to the “treatment,” even though the treatment was only a placebo.