Next, these p-values were converted to Z-scores using a standard normal curve such that p-values below 0.5 were assigned positive Z-scores and p-values above 0.5 were assigned negative Z-scores. Subsequently these Z-scores were combined by calculating ZW=∑i=1kwiZi∑i=1kwi2, where the weighting factors wi corresponds to the individual sample sizes, k corresponds to the number of total studies and Zi corresponds to the individual study Z-scores. The outcome of this test, Zw, follows a standard normal distribution and the corresponding probability can be obtained from a standard normal distribution table. We applied this procedure to the overall sample as well as to each of the three study groups.
