第九周 异常检测

版权声明：转载请注明来源，谢谢。 https://blog.csdn.net/luke_sanjayzzzhong/article/details/82559604

Practice Anomaly Detection

• 第四题 下面哪个是对的？

  • When choosing features for an anomaly detection system, it is a good idea to look for features that take on unusually large or small values for (mainly the) anomalous examples.
    (当建立异常检测系统选择特征时，寻找那些特别大或特别小的值作为异常样本，这样是可行的)

  • If you are developing an anomaly detection system, there is no way to make use of labeled data to improve your system.
    (个人见解：当有了label的时候，可以通过训练集得出模型p(x)，然后在cross validation中选择使F1最大的ϵ，从而改进系统)

  • If you have a large labeled training set with many positive examples and many negative examples, the anomaly detection algorithm will likely perform just as well as a supervised learning algorithm such as an SVM.
    (如果训练集的正和负样本相当，直接用监督学习就好了；异常检测适用于正样本很少的情况)

  • If you do not have any labeled data (or if all your data has label y=0), then is is still possible to learn p(x), but it may be harder to evaluate the system or choose a good value of ϵ.
    (个人见解：没有标签的话，当然比较难选择epsilon，因为不知道哪些是异常值)

    选了1和4

    答案对了！

• 关于怎么从图看出sigma

  

  从图中可以看出，从u到右边的最低点，大概是3*sigma。这应该是个大概吧。