Warm-Up

Homework Questions

Using Studies Wisely Section 4.3

Students often confuse the two types of inferences we can make: inferences about a population and inferences about cause and effect. What type of inference/conclusion can be made from a particular study? The answer depends on the design of the study. Scope of Inference

In a census, the individuals who respond are chosen at random from the population of interest. Random sampling avoids bias and produces trustworthy estimates of the truth about the population. The Census Bureau should be safe making an inference about the population based on the results of the survey. Example

In a sleep deprivation experiment, subjects were randomly assigned to the sleep deprivation and unrestricted sleep treatments. Random assignment helps ensure that the two groups of subjects are as similar as possible before the treatments are imposed. If the unrestricted sleep group performs much better than the sleep deprivation group, and the difference is statistically significant, it must be due to the treatments. It that case, they can make inferences about cause and effect. Example

Overview Were individuals randomly assigned to groups? Were individuals randomly selected? Yes No Inference about population: YES Inference about cause and effect: YES Inference about cause and effect: NO Inference about population: NO Overview

Criteria for Causation The association is strong. The association between smoking and lung cancer is very strong. The association is consistent. Many studies of different kind of people in many countries link smoking to lung cancer. Larger values of the explanatory variable are associated with stronger responses. People who smoke more cigarettes per day or who smoke over a longer period of time get lung cancer more often. The alleged cause precedes the effect in time. Lung cancer develops after years of smoking. The alleged cause is plausible. Experiments with animals show that tars from cigarette smoke do cause cancer. Criteria for Causation

On Part B of your AP Questions – look at number 5 On Part B of your AP Questions – look at number 5! This deals with these topics! Homework