Observes individuals and measures variables of interest but does not attempt to influence the responses

• deliberately imposes some treatment on individuals to measure their responses

  •  only source of convincing causation 

  • have explanatory and response variables 

The effects of two variables on a response variable cannot be separated from each other

Lurking variables tamper with results but are unknown completely

• a collection of individuals that treatment is being applied to 

  • rats or monkey

  • if they are humans → subjects

Comparison - used to defeat placebo effect

Control - helps to determine the effectiveness of treatment by attempting to minimize confounding

Randomization - used to combat bias and equally disperse lurking variables among treatment groups

Replication - uses enough experimental units to defeat chance variation (it is not repeating experiments)