Which of the following describes a level 4 evidence?

Level 4 evidence refers to non-experimental studies, which are typically descriptive and observational in nature. A single non-experimental study often involves the collection of data without manipulation of variables, highlighting its reliance on observation rather than controlled experiments. This type of evidence plays an important role in understanding phenomena in real-world settings and can significantly contribute to the body of knowledge, particularly when higher levels of evidence, like systematic reviews or randomized controlled trials, are not available.

Comparing this to the other options: systematic reviews of qualitative studies and systematic reviews of randomized controlled trials represent higher levels of evidence, as they synthesize findings from multiple studies to derive conclusions. The opinion of expert committees is often considered lower on the hierarchy of evidence, typically categorized as level 5 or expert opinion, which lacks the empirical backing that non-experimental studies might provide. Therefore, identifying a single non-experimental study as level 4 evidence aligns with the evidence hierarchy where observational research is classified lower than systematic reviews but higher than expert opinion.