1. Designing Experiments 5.2

2. Vocabulary Experimental Units: the individuals on which the experiment is done Subjects: when the experimental units are humans Treatment: a specific experimental condition that is applied Factors: explanatory variables in an experiment – Value of factors: level Placebo: a “fake” treatment that looks just like the treatment being tested – Placebo effect: response to “fake” treatment

3. Experiments can give good evidence for causation – and allow us to control the effects of lurking variables. Control is the first basic principle of statistical design of experiments

4. Randomization Rely on chance to make the assignment of subjects to treatments – Random Digit Table, or Random Number Generator. Randomization is essential to a good experiment. Use enough experimental units to reduce the chance variation Completely randomized – when all experimental units are allocated at random among all treatments.

5. Principles of Experimental Design Control the effects of lurking variables on the response by comparing two or more treatments Randomize – use impersonal chance to assign experimental units to treatments Replicate each treatment on many units to reduce chance variation in the results

6. Statistically Significant An observed effect so large that it would rarely occur by chance alone.

7. A manufacturer of boots plans to conduct an experiment to compare a new method of waterproofing to the current method. The appearance of the boots is not changed by either method. The company recruits 100 volunteers in Seattle, where it rains frequently, to wear the boots as they normally would for 6 months. At the end of 6 months, the boots will be returned to the company to be evaluated for water damage. – Describe a design for this experiment that uses the 100 volunteers. Include a few sentences on how it would be implemented.