What Is a Primary Outcome?
Looking for the full method? Read the detailed guide: How to Write Research Objectives.
The primary outcome is the main variable that a study is designed to measure to answer its principal question. The study's sample size is usually calculated for it, and the main conclusions are expected to rest on it.
Example
Invented for teaching: In a trial of an exercise programme, the primary outcome is the change in systolic blood pressure from baseline to 12 weeks, measured with a standard device. Other measures such as weight or quality of life are secondary.
Common misunderstanding
Researchers cannot switch the primary outcome after seeing the results without saying so; doing this can mislead readers (selective outcome reporting). A study should have one clearly stated primary outcome, defined in advance with its timing and measurement.
Learn more
See How to Write Research Objectives for how objectives link to outcomes.
References and further reading
- Chan AW, Tetzlaff JM, Altman DG, et al. SPIRIT 2013 statement: defining standard protocol items for clinical trials. Ann Intern Med. 2013;158(3):200-207. doi:10.7326/0003-4819-158-3-201301080-00007
- Schulz KF, Altman DG, Moher D; CONSORT Group. CONSORT 2010 statement: updated guidelines for reporting parallel group randomised trials. BMJ. 2010;340:c332. doi:10.1136/bmj.c332
Related reading
This page is general educational information about research methods. It is not medical, legal or regulatory advice; follow your institution's ethics board and your target journal's instructions.