A meta-analysis is a type of systematic review. Instead of drawing a conclusion from a single study, a meta-analysis looks at numerous studies for the answer.
It pools numerical analyses from studies of similar design. A meta-analysis can also form part of a further systematic review.
A panel of experts usually leads the researchers who carry out a systematic review. There are set protocols for the detailed search and analysis of the medical literature.
A systematic review is considered a high form of evidence. The conclusions contribute to the formation of a medical consensus on the best form of treatment.
The findings also inform policies set by state healthcare systems, such as whether they should fund a new drug.
Conducting a review
A systematic review compares results from a range of studies.
Clinical guidelines lay out the treatment choices to be followed by health care providers and professionals.
These guidelines depend on recommendations made on the basis of systematic reviews of all the available medical evidence.
The method of conducting these reviews is important, because it must ensure the recommendations will result in the best healthcare for patients.
There are step-by-step instructions for conducting systematic reviews.
The Cochrane Library is a collection of systematic reviews that is widely respected by the international medical community. It follows a scientifically rigorous protocol to produce robust reviews.
Producing a review: 8 steps
The steps outlined below are based on those required for the Cochrane Library. They provide a meticulous process through which researchers can synthesize data from a range of studies.
1: Define the research question
Researchers must first decide what research question they need an answer for. The aim could be, for example: "To assess the effects of a new drug for a particular health problem in certain types of people." The question needs to be very specific.
2: Decide which studies to include in the review
This will be partly decided by the research question, but further "eligibility criteria" will define in advance which studies the team will include or exclude. The studies must have a rigorous design, for example, a randomized control trial (RCT).
3: Search for the studies
Step 3 outlines the sources to be consulted, and the search terms used to search for them. In a Cochrane review, specially trained search coordinators do this. The search should also involve attempts to reveal unpublished studies.
4: Select the studies and collect the data
Data are taken from studies that meet the predetermined eligibility criteria. The data may have to come from a variety of formats.
5: Assess the risk of bias in the included studies
This ensures that all the studies reviewed are relevant and reliable. For example, was the randomization in the trial double-blinded? Or was there a risk of bias, for example, in selecting participants for treatment or comparison? It is acceptable to include some studies of a lower quality, as long as the researchers take this kind of bias into account.
6: Analyze the data and undertake meta-analyses
This is the core process of a systematic review, and the main step towards synthesizing conclusions. The previous steps must be complete before carrying out this step.
7: Address any publication bias
Publication bias is when a study is specifically chosen for inclusion, or cherry-picked. This can lead to a misrepresentation of the true effects of treatment.
8: Present the final results of the review
The team publishes the work, with a table showing a summary of findings. Decision makers can use this published outcome.
A systematic review is a synthesis or overview of all the available evidence about a particular medical research question. Based on the evidence currently available, it can give a definitive answer on a particular question about therapy, prevention, causes of disease, or harm.
It is particularly valuable for establishing whether a certain type of drug works and is safe.
By summarizing large bodies of evidence, a systematic review can help busy doctors to understand the latest developments.
A review can indicate how well findings can be applied to everyday practice. This is known as the generalizability of findings. It can also identify knowledge gaps that call for more research.
The conclusions of reviews are more reliable than those of individual studies. Consulting a review removes the need to try and understand the differences between results from various items of research.
A systematic review minimizes bias when scientists reach conclusions. The mathematical power and precision are high.
Systematic reviews also offer practical advantages. They are less costly to carry out than a new set of experiments, and they take less time.
A systematic review may have some disadvantages.
If the researchers only use published or readily available studies, the conclusions may be unreliable.
Unpublished studies can be hard to find, but using published literature alone may lead to misrepresentation, because it does not include findings from all the existing research.
Results that are negative or inconclusive, for example, may remain unpublished. Publication bias can cause positive results to become exaggerated, because neutral or negative results are suppressed.
Normally, bad news is more likely to hit the headlines than good news. Conversely, medical researchers are less likely to submit bad results, so systematic reviews could have a bias towards good results.
The decisions of journal editors and peer reviewers can also lead to publication bias.
Sometimes, results do not reach the publication stage because there is funding for research, but this does not cover the cost of analyzing and publishing the results. This can limit the motivation to write up and submit any negative or neutral findings for publication.
What is a meta-analysis?
A systematic review answers a defined research question by collecting and summarizing all empirical evidence that fits pre-specified eligibility criteria.
A meta-analysis uses statistical methods to summarize the results of these studies.
A meta-analysis can stand alone, or it can be part of a wider systematic review. It provides reliable evidence. Wider reviews include results from studies of various scientific designs. A meta-analysis includes studies of similar design only.
A meta-analysis uses statistical analysis to combine the numbers from individual studies. It then computes an overall quantitative result.
Even so, the results are not always directly applicable to the everyday treatment of disease. This is because simple numerical answers cannot solve complex clinical problems. Health care professionals must still interpret the findings cautiously.
Wider statistical reviews, too, have their faults. A review may conclude that antibiotics are effective in treating a disease, without defining the type, dosage, and so on.