“…Only 29 studies ( 19 – 21 , 23 , 25 , 26 , 29 – 33 , 36 – 38 , 40 , 43 , 45 , 47 – 50 , 52 , 54 , 55 , 58 , 60 – 63 ) had independent validation sets. Among them, 11 studies ( 21 , 30 , 31 , 33 , 36 , 45 , 55 , 61 – 63 , 65 ) adopted the method of external validation, and the remaining studies carried out internal validation by random sampling. Nine studies ( 20 – 22 , 31 , 32 , 35 , 45 , 52 ) did not clearly describe the variable screening method and mainly adopted single-factor and multi-factor logistic regression.…”