Study Design 101

Randomized Controlled Trial


A study design that randomly assigns participants into an experimental group or a control group. As the study is conducted, the only expected difference between the control and experimental groups in a randomized controlled trial (RCT) is the outcome variable being studied.



Design pitfalls to look out for

An RCT should be a study of one population only.

Was the randomization actually “random,” or are there really two populations being studied?

The variables being studied should be the only variables between the experimental group and the control group.

Are there any confounding variables between the groups?

Fictitious Example

To determine how a new type of short wave UVA-blocking sunscreen affects the general health of skin in comparison to a regular long wave UVA-blocking sunscreen, 40 trial participants were randomly separated into equal groups of 20: an experimental group and a control group. All participants' skin health was then initially evaluated. The experimental group wore the short wave UVA-blocking sunscreen daily, and the control group wore the long wave UVA-blocking sunscreen daily.

After one year, the general health of the skin was measured in both groups and statistically analyzed. In the control group, wearing long wave UVA-blocking sunscreen daily led to improvements in general skin health for 60% of the participants. In the experimental group, wearing short wave UVA-blocking sunscreen daily led to improvements in general skin health for 75% of the participants.

Real-life Examples

Ensrud, K. E., Stock, J. L., Barrett-Connor, E., Grady, D., Mosca, L., Khaw, K., et al. (2008). Effects of raloxifene on fracture risk in postmenopausal women: The raloxifene use for the heart trial. Journal of Bone and Mineral Research, 23(1), 112-120.

This research studied the effect of raloxifene on fracture risk in postmenopausal women, and found that the women who took raloxifene over the same five year period of time as the women who did not reduced their risk of clinical vertebrate fracture.

Krishna, S., Balas, E. A., Francisco, B. D., & König, P. (2006). Effective and sustainable multimedia education for children with asthma: A randomized controlled trial. Children's Health Care, 35(1), 75-90.

This article reports on the effect of multimedia education for children with asthma. A control group of pediatric patients with asthma was given standard asthma educational resources, while the experimental group of pediatric patients with asthma was given standard resources plus multimedia resources. The study found a reduction in daily symptoms, in emergency room visits, in school days missed, and in days of limited activity in the group given multimedia education resources.

Müller, O., Traoré, C., Kouyaté, B., Yé, Y., Frey, C., Coulibaly, B., et al. (2006). Effects of insecticide-treated bednets during early infancy in an African area of intense malaria transmission: A randomized controlled trial. Bulletin of the World Health Organization, 84(2), 120-126.

The study done in this example was to determine whether the early use of bed nets had an effect on the morbidity rate of infants in rural Burkina Faso. An experimental group of infants used bed nets at birth, and a control group of infants used bed nets at six months of age. Results showed no significant difference in morbidity rates, but use of bed nets from birth was related to lower incidence of both malaria and moderate to severe anemia.

Related Formulas

Related Terms

Now test yourself!

1. Having a volunteer bias in the population group is a good thing because it means the study participants are eager and make the study even stronger.

a) True
b) False

2. Why is randomization important to assignment in an RCT?

a) It enables blinding/masking
b) So causation may be extrapolated from results
c) It balances out individual characteristics between groups.
d) a and c
e) b and c

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