Co-Intervention bias

Patients often receive multiple treatments, in addition to the intervention being studied, which could impact the outcome of interest. Co-intervention bias occurs when different groups receive different co-interventions.

For example, in a randomized, controlled trial studying haloperidol in gastroparesis, a number of alternative anti-emetics and analgesics, such as morphine and ondansetron, were also used, and the amount of these co-interventions was different between the two groups. (Roldan 2017) As a result, it is unclear whether the demonstrated reduction in pain and vomiting was solely due to haloperidol, other whether the other medications explain the difference.

Co-intervention bias is a type of intervention bias.

This post is part of a series of posts on bias in medical research. You can find the whole bias catalogue here.

You can find more evidence based medicine resources here.


Roldan CJ, Chambers KA, Paniagua L, Patel S, Cardenas-Turanzas M, Chathampally Y. Randomized Controlled Double-blind Trial Comparing Haloperidol Combined With Conventional Therapy to Conventional Therapy Alone in Patients With Symptomatic Gastroparesis. Academic emergency medicine. 2017. PMID: 28646590 [full text]

Cite this article as:
Morgenstern, J. Co-Intervention bias, First10EM, June 3, 2018. Available at:
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