refer to http://www.mathsisfun.com/accuracy-precision.html
准确率accuracy 和 精度precision 的不同：
准确率是测量值与实际（真）值的接近程度。
精度是测量值彼此接近的程度。
Examples of Precision and Accuracy:
 |  |  |
| Low Accuracy High Precision | High Accuracy Low Precision | High Accuracy High Precision |
So, if you are playing soccer and you always hit the left goal post instead of scoring, then you are not accurate, but you are precise!
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