User968758 said on 3/12/03 2:01 PM: >One definition of an experiment might be that it is a plan for gathering >data, >designed to give informative answers to the questions of interest. How one >"models" the data to get at these answers differs depending on the >experiment. >One might use a regression model, a survival analysis model, a time series >model, etc.... >
Most generally, we may describe a 'study' as any systematic gathering of data for the purposes of gaining an accurate understanding of a phenomena. An Experiment is simply one type of study. Experiments are very precisely defined and have the following characteristics. 1) Two or more conditions in the study, one of which is considered a 'control' condition. 2) A variable, manipulated by the experimenter, which distinguishes the conditions described above. 3) Subjects in the study are randomly assigned to conditions in the study. That is, each subject which enters the subject has an equal chance of assignment to each condition and one subject's assignment has no influence on another subject's assignment. 4) At least one dependent variable is measured. When some of the above conditions are relaxed, you may still be doing a scientific study, but not necessarily an experiment. For instance, sometimes the groups/conditions are pre-existing and random assignment of individuals to the groups is not possible and these are often called Quasi-Experiments. A classic example of this would be Sex with levels Male and Female. One may still do a good scientific study to distinguish differences between the two groups. One may also recognize that conditions as distinguishing groups may not have meaning, but we may still do a correlational study by measuring more than two variables on a randomly obtained sample. In such a case a relationship between the two variables may be explicated even though no experiment has been conducted. Hope that all helps. Paul . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
