Methods A sample survey based on stratified probability samples of 2 204 students aged 12~21 years old among 11 schools in four districts of Shanghai was carried out.
方法采取分层整群随机抽样的方法，对市内四个区11所学校的2 2 0 4名12～2 1岁的在校学生进行不记名问卷调查。
In the process, we calculate the posterior probability of semantics by unlabeled samples information.
在计算的过程中，使用了未标记样本的信息计算语义出现的后验概率。
The background samples are chosen by thresholding inter-frame differences, and the Gaussian kernel density estimation is used to estimate the probability density function of background intensity.
通过相隔固定的帧差值阅值化得到背景样本值，并采用高斯核密度估计方法计算背景灰度的概率密度函数。
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