Understanding Experiments Lecture 10 Section 3.5 Tues, Sep 19, 2006

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 treatment 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. This is called a blinded experiment. Single-blind experiment – The subjects do not know which group they are in (but the researchers do).

Experimenter Bias Experimenter bias – A distortion of the observations resulting from the experimenter’s expectations. 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 – Neither the subjects nor the researchers know which group is the control group. Then how will they ever know whether the drug was effective?

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