Section 5.2 Designing Experiments

Observational Study - Observes individuals and measures variables of interest but DOES NOT attempt to influence the responses. Experiment – DELIBERATLY imposes some treatment on individuals in order to observe their responses.

Experimental Units – individuals on which the experiment is done. –If the experimental unit is a human being, then they are called subjects. –Treatment – experimental condition applied to units

Purpose – of experiment is to reveal the response of one variable to changes in other variables. –Must distinguish between response and explanatory variables. Factors – the explanatory variable in an experiment. In theory experiments can give good evidence for causation. –Allow us to study specific factors we are interested in while controlling lurking variables.

3 Principles of Experimental Design 1)Control – needed to counter effects of lurking variables, etc. Simplest form of control is comparison. Experiments should control 2 or more treatments in order to avoid confounding the effect of a treatment with other influence

3 Principles of Experimental Design 2) Randomization – subjects assigned to treatments by pure chance (table of random digits). - Creates groups that are similar except for chance variation. - ***

3 Principles of Experimental Design 3) Replication – do experiment on many subjects to reduce chance variation in the results.

Treatment – a specific experimental condition applied to experimental units. Placebo – a dummy treatment that can have no physical effect Control Group – receives dummy treatment –Helps control lurking variables.

5.31 The ability to grow in shade may help pines found in the dry forests of Arizona to resist drought. How well do these pines grow in shade? Investigators planted pine seedlings in a greenhouse in either full light or light reduced to 5% of normal by shade cloth. At the end of the study, they dried the young trees and weighed them. Experimental units? Factors? Treatments? Response Variables?

5.34 Sickle-cell disease is an inherited disorder of the red blood cells that in the United States affects mostly blacks. It can cause severe pain and many complications. Can the drug hydroxyurea reduce the severe pain caused by sickle-cell disease? A study by the National Institutes of Health gave the drug to 150 sickle-cell sufferers and a placebo (dummy medication) to another 150. The researchers then counted the episodes of pain reported by each subject. Experimental units? Factors? Treatments? Response Variables?

Comparative Experiments Experiments should compare treatments rather than attempt to assess a single treatment in isolation –Control and placebo groups –Laboratory experiments often simple design with single treatment. –EX gastric freezing p 292, 5.11

Completely Randomized Experiments All experimental units are allocated at random among the treatments Example 5.12, p295

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Logic of Experimental Design Randomization – produces similar groups Comparative Design – ensures experimental treatments operate equally on all groups Results – differences in response variable must be due to effects of treatment

A strong association in data from well organized data experiments does imply causation. (cause – effect relationship) Statistically significant observation – an observed result too unusual to be an outcome determined by pure chance (more in ch 10) Randomized Comparative Experiments are best means of gaining knowledge about the effects of explanatory variables on a response Examine every experiment with a critical eye, watch for bias

Double – blind experiment – neither the subjects nor the people who have contact with them know which treatment a subject receives. The most serious weakness of experiments is lack of realism Lack of realism can limit the ability to apply the conclusions of an experiment to the settings of greater interest

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. –Used to minimize variation. –Similar to stratified designs Matched Pairs Design – a common form of blocking for comparing two treatments.

38 in class 37,39,40, 44, 50, 52, 54, 57 homework

Section 5.3 – basics of simulation Simulation – the imitation of chance behavior, based on a model that accurately reflects the experiment under consideration –an effective tool for finding likelihoods of complex results once we have a trustworthy model. –Gives us good estimates of probabilities

Steps of Simulation 1)State problem or describe experiment 2)State the assumption 3)Assign digits to represent outcomes 4)Simulate many repetitions