Producing Data, Randomization, and Experimental Design

Goals Identify observational studies versus experiments Design experiments to test hypotheses using appropriate randomization Use the random number tables to assign subjects correctly to experimental groups Define, use, and know the concepts behind all the new vocabulary words

Starting with a research question Often can’t simply study the whole population If you want to know the life expectancy for cancer patients you simply can’t identify all patients and then wait for them to die Use a sample to draw conclusions about the whole

Observational Study vs. Experiment Observational study observes and measures variables of interest. Experiment imposes a treatment in order to observe outcome

New Terminology Population Sample Voluntary response sample Convenience sampling Bias Simple random sample (SRS)

Use Random Numbers to Generate SRS Label all the individuals in a population with numerical labels. Use random number table (or statistical package) to choose individuals randomly. Example: To divide 100 students into two groups of 50 label them 00 to 99 and go through the table starting at a random line until the first 50 have been chosen for a group.

Other Sampling Designs Probability sample Individuals chosen with some given probability Stratified random sample Population divided into strata and individuals chosen at random from each strata Multistage random sample Sample chosen in a number of stages

Problems Undercoverage: some groups have no chance of being sampled; as in phone polling Nonresponse: individuals chosen cannot or will not participate Response bias: people may lie about illegal or embarrassing behavior; may respond to the questioner Wording of questions may effect the outcome

Designing Experiments Experimental units or subjects Treatment: experimental condition imposed Factors: explanatory variables Level: value of a given factor

Example: Television ads

Comparative Experiments Compare 2 or more groups Use a control group to eliminate confounding and placebo effect A randomized comparative experiment uses comparisons between two (or more) groups and randomization of subjects into treatment groups.

Design of a randomized comparative experiment

Principles of Experimental Design Control effects of lurking variables via comparison of several treatments Randomization to assign units to treatment Replication of experiment on many units to reduce chance variation