1. Understanding Experiments Lecture 10 Section 3.5 Fri, Sep 17, 2004

2. Examples Recall that in an experiment, the researchers may control the levels of the explanatory variables. Recall that in an experiment, the researchers may control the levels of the explanatory variables. Example 3.7, p. 145 – Producing Hard Gears. Example 3.7, p. 145 – Producing Hard Gears. What are the explanatory variables? What are the explanatory variables? What are the levels for each variable? What are the levels for each variable? What are the treatments? What are the treatments? How many units are there? How many units are there? Let’s do it! 3.5, p. 147 – Component Lifetime. Let’s do it! 3.5, p. 147 – Component Lifetime.

3. Designing an Experiment Treatment group – A group that receives one of the actual treatments. 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 group. Control group – A group that receives no treatment, but is otherwise identical to the treatment group.

4. Example Think About It, p. 149. Think About It, p. 149. What is wrong with this experiment? What is wrong with this experiment? Suppose the researchers decide to divide the 100 subjects into two groups. Suppose the researchers decide to divide the 100 subjects into two groups. Group 1 receives Vitamin C. Group 1 receives Vitamin C. Group 2 does not receive Vitamin C. Group 2 does not receive Vitamin C. How should they be divided? How should they be divided?

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

6. Response Bias Are the subjects in the control group aware that they are not receiving Vitamin C? Are the subjects in the control group aware that they are not receiving Vitamin C? Are they aware of the purpose of the experiment? Are they aware of the purpose of the experiment? If so, will they be more likely to report cold symptoms? If so, will they be more likely to report cold symptoms? Will the treatment group be less likely? Will the treatment group be less likely? What can we do about this? What can we do about this?

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

8. 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. To help eliminate response bias, the researchers could give a similar tablet, but containing no Vitamin C, to the subjects in the control group. Placebo – Something that looks like the real drug or treatment, but in fact has no effect. Placebo – Something that looks like the real drug or treatment, but in fact has no effect.

9. Blinded Experiments To reduce or eliminate response bias, we can use a blinded experiment. To reduce or eliminate response bias, we can use a blinded experiment. Single-blind experiment – The subjects do not know which group they are in. Single-blind experiment – The subjects do not know which group they are in.

10. Experimenter Bias and Double-Blind Experiments Experimenter bias – A distortion in the observations resulting from the experimenter’s expectations. Experimenter bias – A distortion in 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. This is similar to response bias except that it is caused by the experimenter, not the subject. Double-blind experiment – Neither the subjects nor the researchers know which group is the control group. Double-blind experiment – Neither the subjects nor the researchers know which group is the control group.

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

12. Let’s Do It! Let’s do it! 3.7, p. 155 – Study Suggests Light to Back of Knees Alters Master Biological Clock. 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 there a control group? Was this a blinded study? Was this a blinded study? Could there be a placebo effect? Could there be a placebo effect? Let’s do it! 3.8, p. 156 – Controlling for Another Factor. Let’s do it! 3.8, p. 156 – Controlling for Another Factor. Discuss the flaw(s) in this experiment. Discuss the flaw(s) in this experiment.