2.3 Data extraction and management
All authors involved in this study had previously piloted the form on a random sample of five included COSs to ensure the agreement among the interpretation of data items. One reviewer (J.Y.S., M.L.Y., or Z.W.S.) extracted data from the included COSs using a data extraction sheet from included COSs, and a second reviewer (Y.G., Y.M.C., or M.M.N) verified the extracted data. Any disagreements were adjudicated by the third reviewer (J.H.T). The extracted data included: the number of authors, countries which co-authors came from, whether the COS protocol was registered, published journals and impact factors, number of databases searched (Chinese, English, or both), funding source (industry, government, unfunded, or not reported), and impact factors of journals in which COSs included were retrieved by searching 2018 Journal Citation Reports (Thomson Reuters, 2018) via Web of Science. Detailed scope of data extraction tables is shown in Appendix Table 1.