# Situation: Staff nurses on an adult surgical ward want to know whether chewing gum after bowel surgery speeds the return of bowel function. They plan to search the literature and present the evidence to the ward's practice council. The search yields four sources on the question. Which source gives the STRONGEST level of evidence for changing practice?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=629964  
> language: ko  
> subject: Nursing Practice III — Care of Clients with Problems in Surgery, Oxygenation, Fluid and Electrolytes, Infectious, Inflammatory and Immunologic Response, Cellular Aberrations

## 문제

Situation: Staff nurses on an adult surgical ward want to know whether chewing gum after bowel surgery speeds the return of bowel function. They plan to search the literature and present the evidence to the ward's practice council.

The search yields four sources on the question. Which source gives the STRONGEST level of evidence for changing practice?

## 보기

1. A systematic review of 12 prospective cohort studies
2. A prospective cohort study of 5,000 clients in one hospital
3. A multicenter randomized controlled trial of 600 clients **✔ 정답**
4. A consensus statement from a national panel of surgeons

**정답: 3**

## 해설

In the hierarchy of evidence for interventions, systematic reviews and meta-analyses of randomized controlled trials rank highest, then randomized controlled trials, then cohort and case-control studies, with expert opinion lowest. A systematic review ranks at the top only when it pools randomized trials; a review of cohort studies remains observational evidence and ranks below a well-conducted randomized trial. A large sample improves precision but does not change the level of a design.

## 심화 해설

Evidence hierarchy for intervention questions

When a clinical question asks whether an *intervention*—such as chewing gum after bowel surgery—changes an outcome, the strength of evidence depends primarily on the study design, not on sample size or the reputation of the authors. For intervention questions, randomized controlled trials and systematic reviews of randomized trials sit above observational designs and expert opinion.

The four options differ in a way that is deliberately easy to misread. A systematic review sounds impressive, but its position in the hierarchy depends on what it pools. A systematic review of cohort studies is still observational evidence; it cannot produce a stronger causal inference than the primary studies it contains. Option 1 reviews 12 prospective cohort studies. Cohort studies follow groups over time but do not randomly assign the intervention, so confounding by indication, baseline differences, and unmeasured variables can distort the apparent effect of gum chewing.

Option 2 is a single prospective cohort study with a large sample of 5,000 clients. The large number improves precision—the confidence interval narrows—but a bigger sample does not upgrade an observational design to experimental evidence. Precision and validity are separate concepts in evidence appraisal.

Option 4, a consensus statement from a national panel of surgeons, represents expert opinion. Even when experts are highly respected, consensus statements sit near the bottom of the hierarchy for intervention questions because they are not generated through controlled comparison of groups.

Option 3 is a multicenter randomized controlled trial of 600 clients. Randomization balances known and unknown prognostic factors across groups, which is the key mechanism that supports causal inference. The multicenter feature also improves generalizability because the intervention is tested in more than one setting. Key point! For intervention questions, a well-conducted RCT outranks a systematic review of observational studies.

The evidence hierarchy can be summarized as follows:

| Level | Design | Why it ranks there |
| --- | --- | --- |
| Highest | Systematic review or meta-analysis of RCTs | Pools randomized evidence; reduces random error across trials |
| High | Single randomized controlled trial | Randomization supports causal inference |
| Moderate | Systematic review of cohort studies | Still observational; confounding remains possible |
| Low | Single cohort or case-control study | No random assignment; bias risk is higher |
| Lowest | Expert opinion or consensus statement | No controlled comparison of groups |

The provided evidence base reinforces this reasoning. The strongest available sources on gum chewing and postoperative recovery are meta-analyses of randomized controlled trials [1][3][4], and an individual RCT [2]. These align with the principle that randomized evidence—whether pooled or in a single well-designed trial—provides the most trustworthy answer for an intervention question. A review of cohort studies, regardless of how many cohorts it includes, remains a lower level of evidence because the design cannot rule out confounding the way randomization can. Watch out! Do not equate “systematic review” with “highest evidence” automatically; always check whether the review pooled randomized trials or observational studies.References (research sources)

- [1]The Effects of Gum Chewing in the Postoperative-Period: A Systematic Review and Meta-Analysis.Meta-analysis/systematic reviewLi C, Nair A, Zhu XM, Leung M, Leung G, Catapano J (2025) · DOI: 10.1111/ans.70238

- [2]Effect of Chewing Gum on Postoperative Gastrointestinal Recovery in Patients Undergoing Abdominal Surgery: A Randomized Controlled Study.RCT/clinical trialLn AK, Saran S, Ranjan R. (2026) · DOI: 10.7759/cureus.106979

- [3]Chewing gum improves postoperative recovery of gastrointestinal function after cesarean delivery: a systematic review and meta-analysis of randomized trials.Meta-analysis/systematic reviewCiardulli A, Saccone G, Di Mascio D, Caissutti C, Berghella V (2018) · DOI: 10.1080/14767058.2017.1330883

- [4]Chewing gum for intestinal function recovery after caesarean section: a systematic review and meta-analysis.Meta-analysis/systematic reviewWen Z, Shen M, Wu C, Ding J, Mei B (2017) · DOI: 10.1186/s12884-017-1286-8

## 임상 시나리오

Evidence Hierarchy for Intervention QuestionsDesign trumps sample size and author reputation
For questions about whether an intervention changes an outcome, the study design—not the sample size or the prestige of the authors—determines the level of evidence. Randomized controlled trials and systematic reviews of RCTs rank above observational designs and expert opinion.

A systematic review is only as strong as the studies it pools. A review of cohort studies remains observational evidence and cannot produce stronger causal inference than the primary studies it contains. A single cohort study with 5,000 participants improves precision but does not upgrade an observational design to experimental evidence.

CautionDo not assume that a systematic review automatically provides the highest level of evidence—check whether it pools randomized trials or observational studies first. Consensus statements from expert panels, however respected, sit near the bottom of the hierarchy for intervention questions.

## 핵심 개념

- **Evidence hierarchy** — A ranking of study designs by their ability to minimize bias and support causal inference; for intervention questions, systematic reviews of RCTs and RCTs rank highest, followed by observational studies, with expert opinion lowest.
- **Randomized controlled trial (RCT)** — An experimental design in which participants are randomly assigned to intervention or control groups, minimizing confounding and allowing stronger causal inference about treatment effects.
- **Systematic review** — A structured literature review that pools findings from multiple studies; its evidence level depends on the designs of the included studies, not on the label 'systematic review' itself.
- **Cohort study** — An observational design that follows groups over time without random assignment; subject to confounding by indication and baseline differences.
- **Confounding by indication** — A bias in observational studies where the reason for receiving an intervention is associated with the outcome, distorting the apparent treatment effect.

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