Study Design 101: Systematic Review

Definition

A document often written by a panel that provides a comprehensive review of all relevant studies on a particular clinical or health-related topic/question. The systematic review is created after reviewing and combining all the information from both published and unpublished studies (focusing on clinical trials of similar treatments) and then summarizing the findings.

Advantages

• Exhaustive review of the current literature and other sources (unpublished studies, ongoing research)
• Less costly to review prior studies than to create a new study
• Less time required than conducting a new study
• Results can be generalized and extrapolated into the general population more broadly than individual studies
• More reliable and accurate than individual studies
• Considered an evidence-based resource

Disadvantages

• Very time-consuming
• May not be easy to combine studies

Design pitfalls to look out for

Studies included in systematic reviews may be of varying study designs, but should collectively be studying the same outcome.

Is each study included in the review studying the same variables?

Some reviews may group and analyze studies by variables such as age and gender; factors that were not allocated to participants.

Do the analyses in the systematic review fit the variables being studied in the original studies?

Fictitious Example

Does the regular wearing of ultraviolet-blocking sunscreen prevent melanoma? An exhaustive literature search was conducted, resulting in 54 studies on sunscreen and melanoma. Each study was then evaluated to determine whether the study focused specifically on ultraviolet-blocking sunscreen and melanoma prevention; 30 of the 54 studies were retained. The thirty studies were reviewed and showed a strong positive relationship between daily wearing of sunscreen and a reduced diagnosis of melanoma.

Real-life Examples

Yang, J., Chen, J., Yang, M., Yu, S., Ying, L., Liu, G., ... Liang, F. (2018). Acupuncture for hypertension. The Cochrane Database of Systematic Reviews, 11(11), CD008821. https://doi.org/10.1002/14651858.CD008821.pub2

This systematic review analyzed twenty-two randomized controlled trials to determine whether acupuncture is a safe and effective way to lower blood pressure in adults with primary hypertension. Due to the low quality of evidence in these studies and lack of blinding, it is not possible to link any short-term decrease in blood pressure to the use of acupuncture. Additional research is needed to determine if there is an effect due to acupuncture that lasts at least seven days.

Parker, H.W. and Vadiveloo, M.K. (2019). Diet quality of vegetarian diets compared with nonvegetarian diets: a systematic review. Nutrition Reviews, https://doi.org/10.1093/nutrit/nuy067

This systematic review was interested in comparing the diet quality of vegetarian and non-vegetarian diets. Twelve studies were included. Vegetarians more closely met recommendations for total fruit, whole grains, seafood and plant protein, and sodium intake. In nine of the twelve studies, vegetarians had higher overall diet quality compared to non-vegetarians. These findings may explain better health outcomes in vegetarians, but additional research is needed to remove any possible confounding variables.

Related Terms

Cochrane Database of Systematic Reviews

A highly-regarded database of systematic reviews prepared by The Cochrane Collaboration, an international group of individuals and institutions who review and analyze the published literature.

Exclusion Criteria

The set of conditions that characterize some individuals which result in being excluded in the study (i.e. other health conditions, taking specific medications, etc.). Since systematic reviews seek to include all relevant studies, exclusion criteria are not generally utilized in this situation.

Inclusion Criteria

The set of conditions that studies must meet to be included in the review (or for individual studies - the set of conditions that participants must meet to be included in the study; often comprises age, gender, disease type and status, etc.).