Designing Experiments 1

Definitions: Observational study - observes individuals and measures variables of interest without imposing any treatment When goal is to understand cause and effect, experiments are the ONLY source of fully convincing data. Experiment - actively imposes some treatment in order to measure the response

100% of daily vitamins & essential oils! I’ve developed a new rabbit food, Hippity Hop. Makes fur soft & shiny! Hippity Hop Rabbit Food Increases energy! 100% of daily vitamins & essential oils!

Can I just make these claims? NO What must I do to make these claims? Do an experiment Who (what) should I test this on? Rabbits What do I test? The type of food

Factor – is the explanatory variable – it’s what we test Response variable – measures the outcome of a study 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.

Does eating dinner with their families improve students’ academic performance? According to an ABC News article,“Teenagers who eat with their families at least five times a week are more likely to get better grades in school.”  This finding was based on a sample survey conducted by researchers at Columbia University. Was this an observational study or an experiment? Justify your answer. An observational study. Students were not assigned to a particular number of meals to eat with their family per week. What are the explanatory and response variables Explanatory variable: the number of meals per week eaten with their family. Response variable: probably their GPA.

Does eating dinner with their families improve students’ academic performance? According to an ABC News article,“Teenagers who eat with their families at least five times a week are more likely to get better grades in school.”  This finding was based on a sample survey conducted by researchers at Columbia University. Confounding occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other. Every lurking variable is a potential confounding variable. We’ll learn later how to control them.  Explain clearly why such a study cannot establish a cause-and-effect relationship. Suggest a lurking variable that may be confounded with whether families eat dinner together. This is an observational study and there may well be lurking variables that are actually influencing the response variable. For instance, families that eat more meals together may also be families where the parents show more interest in their children’s education and therefore help them to do better in school.

Experimental unit – the single individual (person, animal, plant, etc Experimental unit – the single individual (person, animal, plant, etc.) to which the different treatments are assigned. (When the units are humans, they are called subjects) Treatment – a specific experimental condition applied to the units Factor – is the explanatory variable – it’s what we test Level – a specific value for the factor

A Louse-y Situation A study published in the New England Journal of Medicine (March 11, 2010) compared two medicines to treat head lice: an oral medication called ivermectin and a topical lotion containing malathion. Researchers studied 812 people in 376 households in seven areas around the world. Of the 185 randomly assigned to ivermectin, 171 were free from head lice after two weeks compared to only 151 of the 191 households randomly assigned to malathion. Problem: Identify the experimental units, explanatory and response variables, and the treatments in this experiment. Solution: The experimental units are the households, not the individual people, since the treatments were assigned to entire households, not separately to individuals within the household. The explanatory variable is type of medication and the response variable is whether the household was lice-free. The treatments were ivermectin and malathion.

Example 1: A farm-product manufacturer wants to determine if the yield of a crop is different when the soil is treated with three different types of fertilizers. Fifteen similar plots of land are planted with the same type of seed but are fertilized differently. At the end of the growing season, the mean yield from the sample plots is compared. Experimental units? Factors? Levels? Response variable? How many treatments? Plots of land Type of fertilizer Fertilizer types A, B, & C Yield of crop 3

Example 2: A consumer group wants to test cake pans to see which works the best (bakes evenly). It will test aluminum, glass, and plastic pans in both gas and electric ovens. Experiment units? Factors? Levels? Response variable? Number of treatments? Cake batter Two factors - type of pan & type of oven Type of pan has 3 levels (aluminum, glass, & plastic & type of oven has 2 levels (electric & gas) How evenly the cake bakes 6

Why is the same type of seed used on all 15 plots? Example 3: A farm-product manufacturer wants to determine if the yield of a crop is different when the soil is treated with three different types of fertilizers. Fifteen similar plots of land are planted with the same type of seed but are fertilized differently. At the end of the growing season, the mean yield from the sample plots is compared. Why is the same type of seed used on all 15 plots? What are other potential extraneous variables? Does this experiment have a placebo? Explain It is part of the controls in the experiment. To control the factor of type of seed. Type of soil; amount of water, sunlight, etc. No, one would compare the three types of fertilizers Type of soil, amount of water, etc. NO – a placebo is not needed in this experiment

(a) identify the explanatory and response variables, and TV Advertising What are the effects of repeated exposure to an advertising message? The answer may depend on both the length of the ad and on how often it is repeated. An experiment investigated this question using undergraduate students as subjects. All subjects viewed a 40-minute television program that included ads for a digital camera. Some subjects saw a 30-second commercial; others, a 90-second version. The same commercial was shown either 1, 3, or 5 times during the program. After viewing, all the subjects answered questions about their recall of the ad, their attitude toward the camera, and their intention to purchase it. (a) identify the explanatory and response variables, and (b) list all the treatments. SOLUTION: (a) This experiment has 2 explanatory variables (factors): length of the commercial and number of repetitions. The response variables include measures of subjects’ recall of the ad, their attitudes about the digital camera, and whether they intend to purchase it. (b) There are 2 different lengths of commercial (30 and 90 seconds)and three different numbers of repetitions (1, 3, and 5). The 6 combinations consisting of one level of each factor form the 6 treatments: (1) 30 seconds, 1 time; (2) 30 seconds, 3 times; (3) 30 seconds, 5 times; (4) 90 seconds, 1 time; (5) 90 seconds, 3 times; (6) 90 seconds, 5 times