1. Study Design Data

2. Types of studies Design of study determines whether: –an inference to the population can be made –causality can be inferred random unit selection vs. self-selected units observational study vs. randomized experiment

3. Observational Studies Units inherently belong to groups (beyond the control of the researcher). Can make comparisons between groups, but cannot establish causation. –Correlation does not prove causation! –Confounding variables

4. Randomized Experiment Units are randomly assigned to treatment groups. –A statistically significant difference between treatment groups would provide evidence for causality –Problems: Experimenter bias Experimental subject expectations

5. Why do an observational study? 1. Establishing causation is not necessarily the goal. 2. Establishing causation can be done in other ways. 3. Results may identify potential causal relationships that can be investigated in future studies.

6. Inference to the population can be made Causal inference can be made Selection of Units Random Not random Allocation of Units to Groups Not random (Obs)Random 1 34 2

7. References Ramsey & Schafer. 1997. The Statistical Sleuth. Duxbury Press. Carey. 2004. A Beginner’s Guide to the Scientific Method. Thompson-Wadsworth.