Understanding Experiments Lecture 10 Section 3.5 Mon, Jan 31, 2005

Examples Recall that in an experiment, the researchers may control the levels of the explanatory variables. Example 3.7, p. 145 – Producing Hard Gears. What are the explanatory variables? What are the levels for each variable? What are the treatments? How many units are there?

Let’s do it! Let’s do it! 3.5, p. 147 – Component Lifetime.

Designing an Experiment 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.

Example Think About It, p. 150. What is wrong with this experiment? Suppose the researchers decide to divide the 100 subjects into two groups. Group 1 receives Vitamin C. Group 2 does not receive Vitamin C. How should they be divided?

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 Group 1. The rest would be assigned to Group 2.

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

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, will they be more likely to report cold symptoms? Will the treatment group be less likely? What can we do about this?

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. Placebo – Something that looks like the real drug or treatment, but in fact has no effect.

Blinded Experiments 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 (but the researchers do).

Experimenter Bias and Double-Blind Experiments 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. Double-blind experiment – Neither the subjects nor the researchers know which group is the control group.

Placebo Effect 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.

Example Revisit the study The Role of Cyclooxygenase-2 Inhibitors in Fusion Rates of Hindfoot Arthrodesis Was it a blinded experiment? Single-blind? Double-blind? Did they use a placebo?

COX-2 Study Summary of Methods/Results:  200 consecutive patients undergoing subtalar and ankle arthrodesis were reviewed.  Patients underwent the procedure by either of the senior two authors between 1999 and 2002.  The surgical techniques were performed in similar manner minimizing variability.   One group of patients received COX-2 inhibitor medications postoperatively.  This group received the recommended dose for post-operative pain control for five days after surgery. The other group did not receive this medication.

COX-2 Study Records were reviewed to evaluate for time immobilized, time to full weight bearing, time to radiographic fusion and clinical union.  Radiographs were reviewed to evaluate for radiographic union.   Reviewers were blinded as to which group did and did not receive COX-2 inhibitor medications.

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