An observational study is one in which no variables can be manipulated or controlled by the investigator.
The Clofibrate Trial was a placebo-controlled study to determine the safety and effectiveness of drugs treating coronary heart disease in men.
A confounding variable is an extraneous variable in a statistical model that correlates with both the dependent variable and the independent variable.
The Berkeley study is one of the best known real life examples of an experiment suffering from a confounding variable.
The Salk polio vaccine field trial incorporated a double blind placebo control methodolgy to determine the effectiveness of the vaccine.
Numerous studies have been conducted to examine the value of the portacaval shunt procedure, many using randomized controls.
A scientific control is an observation designed to minimize the effects of variables other than the single independent variable.