There was significant difference between two groups by using Student t test(P<0.05).
经配对t检验表明二者间有显著性差异（P<0.05）；
The mean difference of the post-test was significant at the 0.05 level.
实验组和对比组时间的平均数差异经过t检验，在0.05的水平上具有显著性。
The positive rate of PPD test was no significant difference among each year.
各年度结核菌素阳性率差异无显著性；
The positive rate of PPD test was no significant difference among each year.
各年度结核菌素阳性率差异无显著性；
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