Rivedi, Doll, and Khaw (2003) conducted an experiment to examine whether vitamin D supplementation improved bone health in men and women over 65. A total of 2,686 people aged 65–85 years living in the general community were randomly assigned to receive either oral vitamin D3 supplementation or a matching placebo (a look-alike pill with no pharmacological effect) every four months over five years. Those in the vitamin D group were less likely to have bone fractures than those in the placebo group. [Source: Trivedi, D. P., Doll, R., & Khaw, K. T. (2003). Effect of four monthly oral vitamin D3 (cholecalciferol) supplementation on fractures and mortality in men and women living in the community: randomised double blind controlled trial. British Medical Journal, 326(7387), 469–475.] In this experiment, the independent variable iswhether or not a person had bone fractures , and the dependent variable ispill type .
The individuals who are in the control condition are typically referred to as the control group. Here, the control group consists ofpeople who received the placebos .
is a construct that is being tested in this experiment, and the experimenters have operationally defined it as .