STAT 135 LAB 3

1. STAT 135 LAB 3 TA: Dongmei Li

2. Announcements • HW 1 is due Thursday this week. • Lab 1 and Lab 2 is due Thursday this week (staple the Lab materials with Lab report sheet together).

3. Lecture Review • Response variable • a variable that measures an outcome or result of a study • Explanatory variable • a variable that we think explains or cause changes in the response variable. • Treatment • any specific experimental condition applied to the subjects. • If an experiment has several explanatory variables, a treatment is a combination of specific values of these variables.

4. Example • People who eat lots of fruits and vegetables have lower rates of colon cancer than those who eat little of these foods. Fruits and vegetables are rich in “antioxidants” such as vitamin A, C, and E. Will taking antioxidants help prevent colon cancer? A clinical trial studied this question with 864 people who were at risk for colon cancer. The subjects were divided into four groups: daily beta-carotene, daily vitamins C and E, all three vitamins every day and daily placebo. After four years, the researchers were surprised to find no significant difference in colon cancer among the groups.

5. Questions • What are the explanatory variable and response variables in this experiment? • Answer: antioxidants is the explanatory variable • rates of colon cancer is response variable. • What is the treatment in this experiment? • The treatments are daily beta-carotene, daily vitamins C and E, all three vitamins every day and daily placebo • Are there some lurking variables in this experiment? • Yes. The lurking variables might be gender, age, the life style of those patients, etc.

6. Well-designed experiment • Control the effects of lurking variables (ex: control group, blinding) • Randomization • Use enough subjects to reduce chance variability

7. Another example • An expert on agriculture want to study the effect of fertilizer and soil type on the effect of corn yield. There are two kinds of soil types in this study: black soil and red soil. For fertilizer, there are 3 concentrations (1 lb/square meters, 2 lb/square meters and 3 lb/square meters ).

8. Questions and Answers: • What is the explanatory variable and response variable in this study? • The explanatory variables are fertilizer and soil type. • The response variable is the corn yield. • What are the levels of explanatory variables? • For fertilizer, there are 3 levels: 1 lb/square meters, 2 lb/square meters, and 3 lb/square meters. • For soil type, there are 2 levels: black soil and red soil. • What could be the treatment? • The treatment could be: 1 lb/square meters with black soil, 1 lb/square meters with red soil, 2 lb/square meters with black soil, 2 lb/square meters with red soil, 3 lb/square meters with black soil, 3 lb/square meters with red soil.

9. Lab 3 learning objectives • 14. Learn how to recognize and contrast common problems in experimentation: inability to generalize to relevant populations; lack of applicability to the "real world," bias from unblinded methodology, or a lack of attention to details.