The evidence pyramid: which studies count most
Not all studies are equal. Here is how scientists rank evidence, and how VALREXA’s four evidence levels fit in.
VALREXA Biosciences · 7 min read · November 2026
- Study designs differ in how well they can show cause and effect.
- Randomized controlled trials and systematic reviews sit near the top; cell and animal studies near the bottom for human questions.
- A single study, at any level, rarely settles a question.
The levels, from bottom to top
| Design | What it is | Main limitation |
|---|---|---|
| Cell (in vitro) studies | Experiments on cells in a dish | Cells in a dish are not a living person |
| Animal studies | Experiments in mice, rats or other animals | Results often fail to carry over to humans |
| Case reports and case series | Descriptions of one or a few patients | No comparison group |
| Cross-sectional studies | A snapshot of a population at one time | Cannot show which came first |
| Case-control studies | Compare people with and without a condition, looking back | Prone to recall bias |
| Cohort studies | Follow groups forward over time | Confounding: hidden factors may explain results |
| Randomized controlled trials (RCTs) | Participants randomly assigned to treatment or control | Expensive; may be short or narrow |
| Systematic reviews and meta-analyses | Combine all good-quality studies on a question | Only as good as the studies they include |
Why randomization matters
In an observational study, people who choose a treatment may differ from those who don’t: they may be healthier, wealthier or more health-conscious. Those hidden differences are called confounders. Random assignment spreads them evenly between groups, so a difference in outcome is more likely caused by the treatment itself. Blinding, where participants and researchers don’t know who got what, protects against expectations shaping the results.
Correlation is not causation
Two things rising together does not mean one causes the other. Ice cream sales and drowning both rise in summer because of a third factor, hot weather. Many health headlines report correlations from observational data as if they were causes.
Grading the whole body of evidence
Organizations such as Cochrane use the GRADE system to rate confidence in a conclusion as high, moderate, low or very low, considering study design, consistency, precision and risk of bias.
How VALREXA’s levels map on
| VALREXA level | Meaning |
|---|---|
| Preclinical | Cell and animal studies only |
| Early human | Small or uncontrolled human studies |
| Controlled trials | Randomized controlled trials in people |
| Approved use | Approved by a regulator for a specific use, based on trial evidence |
Read the full method on our editorial standards page.
Further reading
These are authoritative background resources rather than citations to a specific experimental finding. This guide is educational, not medical advice, and does not endorse the use of experimental compounds.