Chapter 12: Stats Modeling the World. Experiments and Observational Studies

Transcription

1 Chapter 12: Stats Modeling the World Experiments and Observational Studies

2 Who Gets Good Grades? In a study conducted at Mission High School, in California researchers compared the scholastic performance of music students with that of non-music students. Researchers first identified subjects who studied music and then collected data on their past grades. They found that music students had a much higher overall grade point average than the non-music students, 3.59 to Result: As a result of this study and others, many parent groups and educators pressed for expanded music programs in the nation s schools. What do you think? Does this study provide solid evidence? Or are there other possible explanations for the difference in grades? Is there any way to really prove such a conjecture?

3 Observational Studies This researcher tried to show an association between music education and grades using a method known as observational study. Observational Study: Researchers don t assign choices; they simply observe them. Observational studies come in two forms: Retrospective & Prospective

4 Observational: Retrospective Retrospective Study: Researchers identify their subjects then use data that has been collected previously. Has the event of interest already occurred?

5 Observational: Prospective Prospective Study: Identifying subjects in advance and collecting data as events unfold. Is the event of interest occurring in my experiment?

6 Identifying Study Type: Example(pg.306): A larger than-usual number of cats and dogs developed a kidney failure; many died. Initially researchers didn t know why, so they used an observational study to investigate. Question: Suppose you were called on to plan a study seeking the cause of this problem. Would your design be retrospective or prospective? Explain How to think about it: What is the event of interest? Why are the animals getting sick. So our interest is in when and how the animals get sick. Can I identify my subjects in advance and collect data as the event unfolds? We don t know the cause of the illness there s no way to identify which pets will get sick and surveying all pets would be impractical.

7 Identifying Study Type: In a study conducted at Mission High School, in California researchers compared the scholastic performance of music students with that of non-music students. The music students had a much higher overall grade point average than the non-music students, 3.59 to What is the event of interest? We are interested in finding out if music effects scholastic performance Did I identify my subjects in advance and collect data as the event unfolds? No, researchers first identified subjects who studied music and then collected data on their past grades.

8 Observational Studies: Conclusions Observational Studies whether prospective or retrospective can not demonstrate a casual relationship. Ex: Does learning to play an instrument also enhance a person s ability to succeed in school? Observational Studies are valuable for discovering trends and possible relationships. Not for justifying them.

9 Experiments: An experiment is a study design that allows us to prove a cause-and-effect relationship. Only an experiment can justify a claim like, Music lessons cause higher grades.

10 An Experiment: (1) Manipulates factor levels to create treatments. (2) Randomly assigns subjects to these treatment levels. (3) Compares the responses of the subject groups across treatment levels.

11 Experiment Design: Factor: A variable whose levels are manipulated by the experimenter. Response: A variable whose levels are compared across different treatments Experimental Units: Individuals on whom or which we experiment Levels: The specific values that the experimenter chooses for a factor. Treatment: Is a combination of specific levels from all the factors that an experimental unit receives

12 Identifying Experiments: Is diet or exercise effective in combating insomnia? Some believe that cutting out desserts can help alleviate the problem, while others recommend exercise. Forty volunteers suffering from insomnia agreed to participate in a month-long test. Half were randomly assigned to a special no desserts diet; the others continued desserts as usual. Half of the people in each of these groups were randomly assigned to an exercise program, while the others did not exercise. Those who ate no desserts and engaged in exercise showed the most improvement. Experimental Units: Who are we doing the experiment on? Factors: What are we testing for each subject? Dessert (Category 1) Exercise (Category 2) Levels: For each factor what are your categories? Dessert (Category 1) No Dessert Dessert Exercise (Category 2) No Exercise Exercise Treatments: What are all the possible combinations of your factor levels? E Y E N D Y Dessert Exercise Dessert No Exercise D N No Dessert Exercise No Dessert No Exercise

13 #9 pg.(326) In a test of roughly 200 older men and women, those with moderately high blood pressure did worse on tests of memory and reaction time than those with normal blood pressure Is this an observational study or experiment? This is an observational study. The researchers are simply studying traits that exist in the subjects, not imposing new treatments. Is it a retrospective or prospective study? This is a prospective study. The subjects were identified first, then the traits were observed. How were the subjects studied and selected? The subjects are roughly 200 men and women with moderately high blood pressure and normal blood pressure. There is no information about the selection method What is the parameter of interest? The parameters of interest are difference in memory and reaction time scores between those with normal blood pressure and moderately high blood pressure. The nature and scope of the conclusion the study can reach? An observational study has no random assignment, so there is no way to know that high blood pressure caused subjects to do worse on memory and reaction time tests. The most we can say is that there was an association between blood pressure and scores on memory and reaction time tests in this group, and recommend a controlled experiment to attempt to determine whether or not there is a cause-and-effect relationship.

14 Classwork in your notes Pg. 326 #1 #3 #20(a) #10 #7 For #7 #10 for experiments do parts b- e. Homework : Due next Thursday 09/08/2016 -#2 -For problems : #8,#13,#15,#19 experiments do parts b-e