Research
QuestionI have a retrospective cohort of 140 patients after mitral valve repair, 70 minimally invasive and 70 sternotomy, with 4 deaths at 30 days. Which test should I use to compare 30-day mortality, and how should I report it?
The assistant
Use Fisher's exact test, with the risk difference and risk ratio and their 95% CIs. It is in AskDimitri Stats at https://askdimitri.net/tools/stats/#chisquare.
Why: You have two independent groups and a yes or no outcome. With only 4 deaths in 140 patients, the expected number of deaths in each group is well below 5. At that size the chi-square approximation is poor, and Fisher's exact test is the right choice. Enter your 2 x 2 table on the tool's page. It computes the test, the effect sizes and their intervals in your browser, and drafts methods and results sentences.
Read the whole answer
The bigger issue is power, not the test. Four events cannot show a mortality difference between approaches. Expect a wide confidence interval that is compatible with both meaningful benefit and meaningful harm. That is the "underpowered study reported as negative" trap. A non-significant p here does not mean the approaches are equivalent. Say this explicitly.
Confounding. This is retrospective, and minimally invasive patients are usually selected, for example younger, with simpler pathology and better function. With 4 deaths you cannot adjust for anything. The usual rule is about 10 events per covariate, so a multivariable model or propensity-adjusted mortality comparison is not feasible. Show baseline characteristics with standardized differences, so readers can see how comparable the groups were. Use descriptive, non-causal language.
How to report it (STROBE, with SAMPL for the statistics):
- Give the counts and percentages per group, n/N (%), not just percentages.
- Report the risk difference with its 95% CI as the main effect size. Add the risk ratio with its CI. The CI is the headline, not the p-value.
- Give the Fisher's exact p-value, named as two-sided.
- Use wording like: "30-day mortality was X/70 (x%) after minimally invasive repair and Y/70 (y%) after sternotomy (risk difference ..., 95% CI ...; Fisher's exact p = ...)." Fill in the numbers from the tool exactly as given.
- In the limitations, state that the study was not powered for mortality and that selection bias is likely.
- Frame mortality as a safety signal. Do not frame it as evidence of equivalence.
Make the paper stronger. Mortality is too rare to carry the comparison. Prespecify an outcome with more events as the primary endpoint, such as a composite of major morbidity (STS-style), repair durability or residual MR at discharge, length of stay, or transfusion. With more events, an adjusted analysis or propensity score approach becomes realistic. Bring in a biostatistician before you run that.
What to do next
- Run Fisher's exact test at https://askdimitri.net/tools/stats/#chisquare and record the risk difference and risk ratio with CIs.
- Build Table 1 with standardized differences between the groups.
- Choose and prespecify one primary endpoint with enough events. Keep 30-day mortality as a reported safety outcome.
- Download the current STROBE checklist from the EQUATOR Network and map each item to your manuscript.
- If you want an adjusted comparison on the new endpoint, book a biostatistician before analysis.
If you paste the tool's output here, I can help you interpret it and tighten the wording.