## Core Concept
Meta-analysis is a statistical technique for combining the findings from independent studies to identify patterns, discrepancies, or overall effects. It is often used in systematic reviews to synthesize data from multiple studies. The core concept revolves around quantitatively analyzing data from various studies to draw more general conclusions.

## Why the Correct Answer is Right
Since the question and options are not provided, let's discuss the general principles of meta-analysis. A correct statement about meta-analysis would involve its ability to increase the power of studies, resolve uncertainty when reports disagree, improve estimates of the size of the effect, and provide a more accurate estimate of the overall effect size.

## Why Each Wrong Option is Incorrect
Without specific details on the options, let's consider general misconceptions about meta-analysis:

* **Option A:** If a statement claims that meta-analysis can only be used for randomized controlled trials (RCTs), it might be misleading because, although RCTs are a common subject of meta-analysis, the technique can be applied to observational studies as well.
* **Option B:** If an option suggests that meta-analysis does not require a thorough literature search, it would be incorrect because a comprehensive and systematic search is crucial for the validity of the meta-analysis.
* **Option C:** If an option states that meta-analysis can only combine studies with similar outcomes, it might be partially true but does not account for methods like narrative synthesis or the use of different effect measures.
* **Option D:** If an option claims that the results of a meta-analysis are always more reliable than those of a single large study, it could be considered incorrect because the quality of a meta-analysis depends on the quality of the included studies.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the validity of a meta-analysis heavily depends on the quality and heterogeneity of the included studies. Heterogeneity, or variability, among study results can significantly affect the interpretation of meta-analysis results.

## Correct Answer: D.

✓ Correct Answer: A. It is a study in which the units of analysis are populations or groups of people, rather than individuals.