Stat 100 Jan. 25

To Do Read Chapter 5

Key Terms Observational Study = essentially a survey, investigator does not assign any tasks to participants Randomized Experiment = researcher assigns different treatments, tasks or conditions to participants

Experiment or observational study? 100 smokers who want to quit are randomly divided into two groups. Researchers give nicotine patches to one group and placebo patches to the other. Percents not smoking after two months are compared. It’s an experiment – treatments are assigned randomly to the participants

Randomization Randomly dividing participants into “treatment” groups Why randomize? Reduces risk of biased comparison. Random assignment to groups should provide comparable groups. Groups should have about same characteristics in the beginning.

Nicotine patch example Randomly assigned treatment groups should have about the same motivation, same degree of addiction, etc.

Placebo Fake treatment Why use a placebo? In “nicotine patch” example, some will quit even without a “treatment.” Use of placebo allows researchers to better assess how much better nicotine patch is than doing nothing

Placebo Effect The observed effect that some people get better even if they use a placebo. In past, attributed to psychological factors. Another explanation – some get better even without treatment Existence of psychological placebo effect under debate.

Ethics of using a placebo Not always ethical, so not always used Comparison is always desirable, so a “control” group often given the existing standard treatment when a new treatment is evaluated

Blind, double blind Blind = either participant or evaluator doesn’t know what treatment is used Double blind = neither participant nor evaluator knows the treatment assignment Nicotine patch surely was “blind.” Participant wouldn’t know if placebo or nicotine patch is used.

Hawthorne effect Often, results in an experiment are better than what will occur in “real world” use of treatments For instance, more might quit smoking in experiment than in normal real world setting Why? – participation in experiment might provide additional motivation

Example Experiment is done to assess effect of alcohol consumption on driving ability Different groups consume different amounts of alcohol For safety reasons, driving test is done using a driving simulator What difficulties does this present for interpreting results?

Ecological Validity Conditions of experiment should resemble real world conditions Alcohol and driving ability experiment might lack ecological validity – Because simulated, not actual driving is done.

Alcohol and driving, continued Suppose there are 3 alcohol conditions – none, moderate amount, high amount How should participants be assigned to conditions?

Assignment methods Only randomized – randomly divide participants into three groups, each assigned a different alcohol condition Blocked – find three similar individuals, randomize one to each condition, repeat this idea over all participants

Criterion for “Similar” In driving and alcohol experiment, what variables would be used to find “similar” people? What factors affect driving and response to alcohol? Weight, age, gender, drinking habits

Blocks Find three people who are roughly same age, weight, and gender(?). Assign one to each condition Repeat this idea over all participants

Why block? Provides more direct control over the comparability of treatment groups.

Observational studies Covered on Jan. 9, See those notes Note – PowerPoint slides are at course web site Also, see Ch. 5