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Quasi-experimental designs provide a level of evidence that includes a comparison group but without random assignment. This design tries to evaluate an intervention’s effect by comparing outcomes between exposed and non-exposed groups, yet it lacks the randomization that protects against selection bias. Because of that, it offers stronger evidence than purely observational or descriptive studies, but it isn’t as robust as a randomized controlled trial, so it sits at Level 2 in many evidence hierarchies. These designs are common when performing an RCT isn’t feasible due to ethics, cost, or logistics, making them a typical source of Level 2 evidence.

Systematic reviews of randomized trials are usually considered higher-level evidence because they synthesize multiple randomized studies. Case series describe outcomes without a control group, so they’re weaker and not Level 2. Nonexperimental studies include observational designs without randomized or controlled comparisons and generally fall lower in the hierarchy.