Experiments and Observational Studies

1. Experiments and Observational Studies Ch. 13

2. Observational Studies • A study in which the researcher observes individuals and measures variables of interest but does not attempt to influence the responses

3. Experiment • A study in which the researcher imposes some treatment on individuals in order to observe their responses

4. Experiments vs. Observational Studies Experiments Observational Study Random selection Can not determine causation Ethical • Random Assignment • Can determine causation • May not be ethical

5. Types of Observational Studies Retrospective Prospective Forward looking Subjects identified ahead of time and data is recorded throughout the study • Backward looking • Subjects are identified by outcomes and past data is recorded

6. Variables • Factor or (Explanatory variable) – the variables that are controlled by the experimenter • Response variable – the variable that the experimenter thinks may be affected by the explanatory variables

7. Manipulating the Factors • Levels – the specific values that the experimenter chooses for a factor • Treatment -- the combination of specific levels from all the factors

8. What we Experiment On • Experimental units • Subjects • Participants

9. Four Principles of Experimental Design • Control • Randomize • Replicate • Block

10. 1. Control • Try to minimize sources of variation by making conditions between groups as similar as possible

11. 2. Randomize • Experimental units are assigned to treatments at random • Helps spread out sources of variation across treatment groups

12. 3. Replicate • 1st type: Apply each treatment to many experimental units • An experiment on a single subject is garbage • 2nd type: Re-run the experiment

13. 4. Block • Similar to stratification in sampling • A group of experimental units are similar in some way that may affect the response to treatment • The similar trait is known to the researcher ahead of time