+ 2. An educational software company wants to

+ 2. An educational software company wants to

+ 2. An educational software company wants to compare the effectiveness of its computer animation for teaching biology with that of a textbook presentation. The company gives a biology pretest to each of a group of high school juniors, and then divides them into two groups. One group uses the animation, and the other studies the test. The company retests all students and compares the increase in biology test scores in the two groups. Is this an observational study or an experiment? Justify your answer. cancer patient with a person of the same age, sex, and race who did not have brain cancer, then asked about the use of cell phones. Result: Our data suggest that the use of hand-held cellular phones is not associated with risk of brain cancer. Is this an observational study or an experiment? Justify your answer. Exp eri me nts + Section 4.2 Experiments Learning Objectives After this section, you should be able to DISTINGUISH observational studies from experiments

DESCRIBE the language of experiments APPLY the three principles of experimental design DESIGN comparative experiments utilizing completely randomized designs and randomized block designs, including matched pairs design + Observational studies of the effect of one variable on another often fail because of confounding between the explanatory variable and one or more lurking variables. nal Study versus Experiment

Observatio Definition: A lurking variable is a variable that is not among the explanatory or response variables in a study but that may influence the response variable. Confounding occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other. Exp eri me nts Exp eri me nts + What confounding variables could also affect how long a person lives? They proceed to design a study, and set about gathering data. Their results

indeed show that people who drink excessively are likely to die at a younger age. die at a younger age. + An experiment is a statistical study in which we actually do something (a treatment) to people, animals, or objects (the experimental units) to observe the response. Explanatory variables are called factors. Here is the basic vocabulary of experiments. The

Language of Experiment s Definition: A specific condition applied to the individuals in an experiment is called a treatment. If an experiment has several explanatory variables, a treatment is a combination of specific values of these variables. The experimental units are the smallest collection of individuals to which treatments are applied. When the units are human beings, they often are called subjects. Exp eri Sometimes, the explanatory variables in an experiment are called factors. me Many experiments study the joint effects of several factors. In such an nts experiment, each treatment is formed by combining a specific value (often called a level) of each of the factors. Exp eri me nt c) b) a) List all the treatments

Identify the factors and response variable + What are the experimental units? others saw a 90 second coomercial. The same commercial was shown either 1, 2 or 3 times during the program. After viewing, all subjects answered questions about their recall of the ad. Exp eri me nt List all the treatments Identify the

factors and response variable + 2. A clothing manufacturer is setting up a new denim line. To test possible looks, the finishing machine will use either metal or bristle rollers, have a dyeing time of either 30 or 40 minutes and run at a temperature of either 150 or 175 degrees Celcius. How many treatments are there? c) b) + o w to E x p

er i m e nt H Example, page 236 A high school regularly offers a review course to prepare students for the SAT. This year, budget cuts will allow the school to offer only an online version of the course. Over the past 10 years, the average SAT score of students in the classroom course was 1620. The online group gets an average score of 1780. Thats roughly 10% higher than the long- time average for those who took the classroom review course. Is the online course more effective? Exp eri me nt Definition: In an experiment, random assignment means that experimental units are assigned to treatments at random, that is, using some sort of chance process. Exp eri me nts

+ Comparison alone isnt enough, if the treatments are given to groups that differ greatly, bias will result. The solution to the problem of bias is random assignment. The remedy for confounding is to perform a comparative experiment in which some units receive one treatment and similar units receive another. Most well designed experiments compare two or more treatments. + Randomized Comparative Experiment

The Definition: In a completely randomized design, the treatments are assigned to all the experimental units completely by chance. Some experiments may include a control group that receives an inactive treatment or an existing baseline treatment. Group 1 Experimental Units Treatment 1 Compare Exp Results Random Assignment Group 2 Treatment 2 eri me nts Exp eri

me nt + School officials want to determine how effective Project 2400 is for preparing students for the SAT. 30 college-bound juniors were selected and randomly assigned to one of two groups. 15 participated in Project 2400 and the other 15 were left to prepare for the SAT on their own. After the SAT was taken, scores were compared. Draw the experimental design. Example 1 Exp eri me nt + wants to determine

the effect of consumer information on energy conservation. One alternative is to install a digital display inside the house to show electricity usage. A second, cheaper, alternative is to provide a chart to the customer so they can go outside and record usage numbers from the meter. A control group will receive information on energy conservation but no tools. Outline a completely randomized experiment involving 60 families.

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