4.3 using studies wisely
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  • 1. Using Studies Wisely Two types of inference: - inference about the population - inference about cause and effect Randomization helps you decide which type of inference is appropriate!! Random selection of individuals allows for inferences to be made about the population. Random assignment of individuals to groups allows for inferences to be made about cause and effect.
  • 2. Using Studies Wisely Two examples: what can or can’t we infer….??? • The U.S. Census Bureau carries out a monthly Current Population Survey of about 60,000 households. Their goal is to use data from these randomly selected households to determine the percent of unemployed individuals in the population. • Scientists performed an experiment that randomly assigned 21 volunteer subjects to one of two treatment groups: sleep deprivation for one night or unrestricted sleep. The experiment hoped to show that sleep deprivation causes a decrease in performance two days later.
  • 3. Using Studies Wisely Were individuals randomly assigned to groups? YES NO Inference about the population: YES Inference about cause and effect: YES Inference about the population: YES Inference about cause and effect: NO NO Were individuals randomly selected? YES Inference about the population: NO Inference about cause and effect: YES Inference about the population: NO Inference about cause and effect: NO
  • 4. Using Studies Wisely A small town dentist wants to know if a daily dose of 500 mg of vitamin C will result in fewer canker sores in the mouth than taking no vitamin C. Design 1: Get all dentists in town with appointments in the next two weeks to take part in a study. Give each patient a survey with two questions: a) Do you take at least 500 mg of vitamin C each day? b) Do you frequently have canker sores? Based on patients’ answers to Question a, divide them into two groups, those who take at least 500 mg of vitamin C daily and those that don’t. Suppose she compares the proportions of patients in each group who complain of canker sores and she finds statistically significant differences compared to those that take at least 500 mg of vitamin C. What can the dentist conclude?????
  • 5. Using Studies Wisely Design 2: Get all dental patients in town with appointments in the next two weeks to take part in a study. Randomly assign half of them to take 500 mg of vitamin C each day and the other half to abstain from taking vitamin C each day for three months. Suppose she compares the proportions of patients in each group who complain of canker sores and she finds statistically significant differences compared to those that take at least 500 mg of vitamin C. What can the dentist conclude?????
  • 6. Using Studies Wisely Design 3: Select a random sample of dental patients in town and get them to take part in a study. Divide the patients into two groups, those who take at least 500 mg of vitamin C daily and those that don’t. Suppose she compares the proportions of patients in each group who complain of canker sores and she finds statistically significant differences compared to those that take at least 500 mg of vitamin C. What can the dentist conclude?????
  • 7. Using Studies Wisely Design 4: Select a random sample of dental patients in town and get them to take part in a study. Randomly assign half of them to take 500 mg of vitamin C each day and the other half to abstain from taking vitamin C each day for three months. Suppose she compares the proportions of patients in each group who complain of canker sores and she finds statistically significant differences compared to those that take at least 500 mg of vitamin C. What can the dentist conclude?????
  • 8. Using Studies Wisely A well-designed experiment tells us that changes in the explanatory variable causes changes in the response variable. The lack of realism can limit our ability to apply the conclusions of an experiment to the settings of greatest interest. Brake Lights
  • 9. Using Studies Wisely In some cases, it isn’t practical or ethical: - Does texting while driving increase risk of accident? - Does going to church help you live longer? - Does smoking cause lung cancer? So we can’t do an experiment on this idea, so now what…??
  • 10. Using Studies Wisely Criteria for establishing causation when you can’t experiment • Association is strong (smoking & cancer – very strong) • Association is consistent (many countries) • Larger values of the explanatory variable are associated with stronger responses (smoke more or longer, more cancer) • The alleged cause precedes the effect in time (30 years later) • The alleged cause is plausible (experiments with animals) Evidence for causation is overwhelming, BUT it is not as strong as evidence by an experiment