We are often asked why some studies are better than others.
This is a complex subject. Apart from the quality, design, and other factors, we have to take into account the different levels of evidence. As you can see from this chart, expert opinion is the weakest evidence and systematic reviews are the strongest.
For example, Acupuncture as an adjunct to IVF has been researched extensively. The big majority of the studies that used good methodology showed that acupuncture was effective. Systematic reviews have summarised those studies and confirmed that acupuncture was an effective therapy to improve IVF success rate. Therefore, one could say that there is a strong evidence supporting acupuncture as an adjunct to IVF.