Plan for today: Chapter 6: Experiments in the Real World Chapter 5: Experiments, Good and Bad
What Is an Experiment? Recall: Observational study: does not attempt to influence responses. Experiment: impose some treatment on individuals
The Vocabulary of Experiments: A response variable is a variable that measures an outcome or result of a study. An explanatory variable is a variable that we think explains or causes changes in the response variable. A treatment is any specific experimental condition applied to the subjects.
An Example of Experiment: The government wishes to determine if a newly developed drug can cure the patients faster than the traditional one. 100 patents were chosen. Half of them were asked to take the new drug and half of them took the traditional one. The recovery speed was recorded for each patient. Response variable: recovery speed Explanatory variable: if the new drug was applied Treatment: new drug / traditional drug
Confounding: A lurking variable is a variable that has an important effect in a study but is not included in the study. In our example, if the patient’s doctor will get cash reward if the patient took the new drug and recovered faster than usual. Is the result still reliable? Two variables are confounded when their effects cannot be distinguished.
Confounding: New drug Recovery speed Cash reward for doctor Cannot be distinguished ?
Placebo Effect: Harry Potter pretended to add the Felix Felicis (a.k.a “liquid luck“) to Ron’s drinking and Ron later won the Quidditch game perfectly.
Placebo Effect: Many patients respond favorably to any treatment, even a placebo. This response to a dummy treatment is the placebo effect. A placebo is a dummy treatment with no active ingredients.
Placebo Effect : Sometimes, because the placebo effect is strong, experiment with human subjects should be double-blind. In a double-blind experiment, neither the subjects nor the people who work with them know which treatment each subject is receiving.
Completely Randomized Design: In a completely randomized experimental design, all the experimental subjects are allocated at random among all the treatments. Subjects Group 1 Group 2 Group 3 Random assignment Drug 1 Drug 2 Placebo
Block Design: A company wants to conduct an experiment to compare the effectiveness of three television commercials for the same product. The CEO read a news says: women and men respond differently to advertising. So he wants to look separately at the reactions of men and women, as well as assess the overall response to the ads.
subjects Group 1 Group 2 Group 3 Random assignment AD 1 AD 2 AD 3 Completely Randomized Design: Ignored the fact that women and men respond differently to advertising.
subjects women men Group 1 Group 2 Group 3 Group 4 Group 5 Group 6 Random assignment AD 1 AD 2 AD 3 A Better Strategy: Block Design Two blocks
Block Design: In a block design, the random assignment of subjects to treatments is carried out separately with in each block. A block is a group of experimental subjects that known before the experiment to be similar in some way that is expected to affect the response to the treatments.
Another very tricky Design: Matched pairs designs compare two treatments by giving one to each of a pair of similar subjects or by giving both to the same subject in random order. Drug A Drug B Patient 1Patient 2twins Patient 50Patient 49twins …… Drug A Drug B Patient 1 …… Patient 50 Drug B Drug A Type 1: Type 2: