Section 5.2 Designing Experiments AP Statistics October 27 th, 2014

What’s the Difference? Observational Study Passive collection of data Observe but don’t interfere Experiment Actually do something to people, objects, animals, etc Observe a response 2

Terminology experimental units The individuals on which the experiment is done are the experimental units. subjects When the units are human beings, they are called subjects. treatment A specific experimental condition applied to the units is called the treatment.  If experiment has several explanatory variables, a treatment is a combination of these variables

UnitsTreatmentObserve ResponseExperiments The explanatory variables in an experiment are often called factors. Each treatment is formed by combining a specific value (often called a level) of each of the factors.

Medical Experiments The following definitions are specific to experiments that study the effectiveness of medical treatments. These experiments are called clinical trials.

The Placebo Effect “Gastric freezing” is a clever treatment for ulcers in the upper intestine. The patient swallow a deflated balloon with tubes attached, the a refrigerated liquid is pumped through the balloon for an hour. The idea is that cooling the stomach will reduce its production of acid and so relieve ulcers. An experiment reported in the Journal of the American Medical Association showed that gastric freezing did reduce the acid production of and relieve ulcer pain.

The Placebo Effect The “Gastric freezing” experiment was poorly designed. The patients’ response may have been due the placebo effect. A placebo is a dummy treatment. Many patients respond favorably to any treatment, even a placebo. This may be due to trust in the doctor and expectations of a cure, or simply to the fact that medical conditions often improve without treatment.

Basic Experiment A standard experiment on “avoidance conditioning” in rats is as follows. The apparatus is a cage with two compartments separated by a door. One compartment is painted white and is lighted, while the other is black and unlighted. A rat is placed in the black compartment, the door is opened and a bell rings. If the rat is still in the black compartment after 5 seconds, it is shocked through the metal door until it moves to the white compartment. Since the rat prefers the dark, it will not move to the white compartment until it has been conditioned to avoid the shock. The process is repeated to observe how many trials (usually 20 or fewer) are required to condition the rat. Identify the experimental units, treatment and response variable in this experiment. AP Statistics, Section 5.1, Part 1 9

Principles of Experimental Design 1. Control of effects of lurking variables on the response, most simply by comparing several treatments. 1. Refers to the overall effort to minimize variability in the way the experimental units are obtained and treated. 2. Randomization, the use of impersonal chance to assign subjects to treatments. 1. Comparison of the effects of several treatments is valid only when all treatments are applied to similar groups of experimental units. 3. Replication of the experiments on many subjects to reduce chance variation in the results.

Randomized Comparative Experiments One of the most common experimental designs that is based on these principles is a randomized comparative experiment. In a randomized comparative experiment multiple treatments are compared. To control for the effects of lurking variables, a control group is used as part of the experimental design. Let’s take a look at an example….. AP Statistics, Section 5.1, Part 1

The Physicians’ Health Study Does regularly taking aspirin help protect people against heart attacks? The Physicians’ Health Study looked at the effects of two drugs: aspirin and beta carotene. The body converts beta carotene into vitamin A, which may help prevent some forms of cancer. A combination of the drugs were given to 21,996 male physicians.

AP Statistics, Section 5.1, Part 1 13 Name the subjects, factors, levels, and treatments. ] Construct an outline of the design of the study. (When you outline the design of an experiment, be sure to indicate the size of the treatment groups and the response variable.)

What’s the Goal? The goal of an experiment is to see a difference in the responses that is so large that it is unlikely to occur by chance. Statistical Significance: An observed effect so large that it would rarely occur by chance is called statistically significant. AP Statistics, Section 5.1, Part 1 14

Double-Blind Experiment In a double-blind experiment neither the subject or the person in contact with them knows which treatment a subject received if any. Avoids unconscious bias

Block Design A block design is a group of experimental units or subjects that are similar in ways that are expected to affect the response to the treatments. In a block design, the random assignment of units to treatments is carried out separately within each block. Blocks are another form of control

Groups in an Experiment matching Sometimes in experiment they will use matching to create the groups of individuals. Matching is when you try to match the two groups up so that they have the same number of people in different categories such as age, sex, race, etc…

Matching When matching is combined with a block design you create a matched-pairs design.  Compares just two treatments  Choose subjects that are closely matched (Random assignment of treatment for each partner) *Sometimes each “pair” in a matched pairs design consists of just one subject, who gets both treatments one after the other. Each subject serves as his or her own control. The order of the treatments can influence the subject’s response, so we randomize the order for each subject

Women and men respond differently to advertising. An experiment to compare the effectiveness of 3 television commercials for the sample product will want to look separately at the reactions of men and women, as well as assess the overall response to the ads. A completely randomized design considers all subjects, both men and women, as a single pool. The randomization assigns subjects to three treatment groups without regard to gender. This ignores the differences between men and women. A better design considers women and men separately. Outline the design of this experiment by drawing a diagram. Men, Women and Advertising