3.2 Evaluation of Literature Quality
According to the guidelines, we summarised the quality of OXC PPK studies based on 35 items, as outlined in Table 1 . Compared with that of the PPK articles published between 2005 and 2013, the quality of PPK studies published between 2014 and 2019 significantly improved [compliance rate median (range): 71.4% (68.6%–71.4%) vs. 94.3% (85.7%–97.1%)]. Generally, prospective studies show better compliance than retrospective studies; however, the prospective study by Northam et al. [25] had a compliance rate of only 73.5%. This may be due to the lack of a standard reporting guide published at that time.
After the guidelines were published, the quality of PPK study reports improved over time. Nonetheless, several studies still have compliance rate of less than 60%, which included deficiencies in the description of sample, statistical methods and software, and methods for handling missing data.