Experiments

The difference between an experiment and a study is that in an experiment, some sort of treatment is imposed. In a study, the data has no manipulation and is merely observational. The two types of studies are retrospective and prospective. A retrospective study records data afterwards and prospective records data as it unfolds.

**Key Concepts**

 * Observational Study:** draws inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator. This is in contrast with experiments, such as randomized controlled trials, where each subject is randomly assigned to a treated group or a control group before the start of the treatment.
 * Retrospective Study:** generally means to take a look back at events that already have taken place. For example, the term is used in medicine, describing a look back at a patient's medical history or lifestyle.
 * Prospective Study:** that follows over time a group of similar individuals, who differ with respect to certain factors under study, to determine how these factors affect rates of a certain outcome. For example, one might follow a cohort of middle-aged truck drivers who vary in terms of smoking habits, to test the hypothesis that the 20-year incidence rate of lung cancer will be highest among heavy smokers, followed by moderate smokers, and then nonsmokers.
 * Experiment:** methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when a particular factor is manipulated. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results. A child may carry out basic experiments to understand the nature of gravity, while teams of scientists may take years of systematic investigation to advance the understanding of a phenomenon. Experiments can vary from personal and informal (e.g. tasting a range of chocolates to find a favorite), to highly controlled (e.g. tests requiring complex apparatus overseen by many scientists that hope to discover information about subatomic particles). Uses of experiments vary considerably between the natural and social sciences.
 * Random Assignment:** is an experimental technique for assigning subjects to different treatments (or no treatment). The thinking behind random assignment is that by randomizing treatment assignment, then the group attributes for the different treatments will be roughly equivalent and therefore any effect observed between treatment groups can be linked to the treatment effect and is not a characteristic of the individuals in the group.
 * Factor:** is a variable that can take on one of a limited, and usually fixed, number of possible values. Categorical variables are often used to represent categorical data. In computer science and some branches of mathematics, categorical variables are referred to as enumerations or enumerated types. Commonly (though not in this article), the word level is used to refer to one of the possible values of a categorical variable.
 * Response:** the dependent variable is the event studied and expected to change whenever the independent variable is altered.
 * Experimental Units:**
 * Level:**
 * Treatment:**
 * Principles of Experimental Design:**
 * Statistically Significant:**
 * Control:**
 * Blinding:**
 * Placebo Effect:**
 * Placebo:**
 * Block:**
 * Matching:**
 * Designs:**
 * Confounding Variables:**