250这个样本数量从哪里来的啊

问题如下：

A risk analyst is asked to verily a VaR model by checking the rate of exceedances of the model. In the past year, there were eight exceedances of the 99% VaR. Is this evidence sufficient to reject the hypothesis, using a 99% test confidence level, that this particular VaR model is unbiased?

选项：

A.No. because the computed z = 2.20.

B.No. because the computed z = 2.29.

C.Yes, because the computed z = 2.87.

D.Yes, because the computed z = 3.47.

解释：

To verify the accuracy of a VaR model by checking the rate of exceedances, the analyst can use the binomial test, particularly if the sample size is large enough. The Kupiec's Proportion of Failures (POF) test is one such test.

The POF test's test statistic is given by:​