Uncontrolled before-and-after studies
Why this level?
The design family and its defining features have not been specified for this hierarchy. NHMRC 2009 sourceA single series measured before and after is Level IV intervention evidence. A distinct historical comparison is III-3, while a controlled before-and-after design with concurrent controls is III-2. The broad pre/post label is insufficient to choose among them.
Where the NHMRC 2009 hierarchy names this design, it places it here:
| Research question | Level | When |
|---|---|---|
| Intervention | III-2 | An observational comparison uses concurrent controls. Statistical adjustment does not change this design level. |
| Intervention | III-3 | The comparison uses historical controls or separate single-arm studies, without concurrent controls. |
| Intervention | IV | A single case series has post-test or pre-test/post-test outcomes, without a comparison group. |
What question does this help answer?
It is the design most quality improvement work actually uses, and it can be informative about whether anything changed. What it cannot do on its own is establish that the intervention caused the change.
Why it is weak
Anything else that changed at the same time is indistinguishable from the intervention: a secular trend, a seasonal pattern, a concurrent policy, staff turnover, or regression to the mean if the intervention was triggered by an unusually bad period. A single before value and a single after value cannot separate any of these from an effect.
Three stories for the same change
How to strengthen it
Three additions do most of the work. Multiple time points before and after turn it into an INTERRUPTED TIME SERIES, which can distinguish a step change from a pre-existing trend. A comparison site turns it into a DIFFERENCE IN DIFFERENCES. A non-equivalent outcome that the intervention should not affect provides a check on whether something broader was changing.
When to use it
Use it when nothing stronger is available, when the change is being made regardless and the alternative is no evaluation at all, and when the finding will be treated as provisional.
Why Mallard may recommend it
Mallard may accept a before-and-after design when the investigator describes an audit of a change already made, and will normally suggest adding pre-intervention time points or a comparison group where the data allow.
Key assumptions and design requirements
- The assumption that nothing else changed is stated explicitly, because it is the whole design.
- As many pre-intervention time points as the data allow are used, not a single aggregate.
- Regression to the mean is considered whenever the intervention was prompted by a bad result.
- The measurement method is identical before and after, since a change in ascertainment mimics an effect.
- The conclusion is framed as consistent with an effect rather than as demonstrating one.
What to watch out for
Without a concurrent comparison group, any secular trend, seasonal pattern, regression to the mean, or concurrent change is indistinguishable from the intervention's effect. It is the weakest of the common evaluation designs and should be strengthened wherever possible with multiple pre-intervention time points, a control site, or both.
Clinical example
Question: Did a new discharge checklist reduce readmissions? Design: Compare the twelve months before with the twelve months after, but plot monthly rates rather than two averages, and if a comparable ward did not adopt the checklist, use it as a comparison series.
How to report it
No reporting guideline dedicated to the uncontrolled before-and-after study was found in a PubMed search on 2026-09-16. TREND is the closest fit, written for nonrandomised evaluations of an intervention rather than for this design in particular. Its items on the intervention, the comparison and the threats to validity apply; what it cannot supply is a concurrent control, so state what else changed over the same period. A reporting guideline is not a design manual: use it to make the study transparent, not as a substitute for careful design. Check the target journal's own requirements as well, which this page cannot know.
- Segmented Regression in an Interrupted Time Series Study Design (opens in a new tab) · Anesth Analg · doi 10.1213/ANE.0000000000005269
- Overview of clinical study designs (opens in a new tab) · Clin Exp Emerg Med · doi 10.15441/ceem.23.036
- Improving the reporting quality of nonrandomized evaluations of behavioral and public health interventions: the TREND statement (opens in a new tab) · Am J Public Health · doi 10.2105/ajph.94.3.361