4.2 - Experiments

Observational Studies measures variables of interest without attempting to influence the responses. sample surveys watching animals in nature you provide no influence on responses just notices relationship, doesn’t imply causation

Experiment deliberately imposing some treatment(s) on individuals to measure their responses. does the treatment cause a change in the responses? helps determine a cause and effect relationship

Lurking Variable a variable that is not the explanatory or the response variable in a study but may influence the response variable. example: you provide data for the relationship between the number of dinners you eat with your family and your GPA. The number of dinners you eat with your family might not be the only reason for a higher GPA. A lurking variable could be the interest your parents have in your education - therefore your parents involvement is creating a higher GPA, not necessarily the number of dinners.

Confounding occurs when 2 variables are associated in such a way that their effects on a response variable cannot be distinguished from each other. By designing effectively, you can prevent lurking variable from becoming confounding variables. Observational studies often fail due to confounding of explanatory variables and lurking variables.

Parts of an Experiment 1. Explanatory Variable (aka Factors) - we can have more than one 2. Response Variable - what we are measuring as a result of the experiment 3. Treatment - the specific condition applied to individuals in an experiment. Could be 1 or many in one experiment. 4. Experimental Units - the smallest collection of individuals to which treatments are applied. (when they are humans they are called subjects) 5. Levels - when there is more than one treatment option due to multiple factors in an experiment

Identify the experimental units, the explanatory and response variables, and treatments in the following experiments. A study published in the New England Journal of Medicine in March 2010 compared 2 medicines to treat head lice: an oral medication (ivermectin) and a topical lotion (malathion). Researchers studied 812 people in 376 households in 7 areas around the world. Of the 185 households randomly assigned to ivermectin, 171 were free from head lice after 2 weeks compared with only 151 of the 191 households randomly assigned to malathion.

Does adding fertilizer affect the productivity of tomato plants? How about the amount of water? To answer these questions, a gardener plants 24 similar tomato plants in identical pots in his greenhouse. He will add fertilizer to the soil in half the pots. Also, he will water 8 of the plants with 0.5 gallons of water per day, 8 of the plants with 1 gallon of water per day, and the remaining 8 plants with 1.5 gallons of water per day. At the end of three months, he will record the total weight of tomatoes produced by each plant. Identify the experimental units, the explanatory and response variables, and treatments in the following experiments.