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Statistical methods

Procedure

The SPSS 22.0 software was used for statistical analyst, the measurement data were expressed as mean ± standard deviation $x¯±s$, t test was used for comparison between two groups, One-Way ANOVA was used for analysis among three groups, and χ2 test was to compare the enumeration data between groups. The receiver operating characteristic curve (ROC) was adopted to evaluate the sensitivity and specificity of USG in predicting VVS and area under the curve (AUC) indicated the predictive ability of USG. AUC of 0.5 ~ 0.7 indicates low predictive ability, 0.7 ~ 0.9 is moderate predictive ability, and > 0.9 indicates high predictive ability. When the Youden index (the sum of sensitivity and specificity minus 1) is the maximum, the sensitivity and specificity will reach the best, and this node was selected as the cut-off value of the prediction index. P < 0.05 was considered statistically significant in difference.

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