There is currently difficulty in replicating trials studying acupuncture. This is due to the high risk of bias in the majority of trials, as well as the difference in standards and conditions that have been used in each experiment. Additionally, compiled data is considered unreliable as there are differences in the methods through which acupuncture is conducted, the number of needles used, the retention time, the type of stimulation used on the needles, the frequency and the total number of treatments, placement on the body, and more. These differences make it hard to conduct an experiment again to test reliability.