Suppose that I am trying to use the jack-knife to estimate the variance of some estimator $$E$$. If I have $$n$$ data points, I begin by computing $$n$$ estimates (call them $$B_1$$, ..., $$B_n$$), each obtained from leaving one of the $$n$$ data points out of my sample. If I understand it correctly, the Wikipedia page then suggests that my estimate of the variance should be

$$\hat{Var(E)} = \frac{\sum_{i = 1}^n (B_i - \bar{B_i})^2}{n-1}$$

where $$\bar{B_i}$$ is the average of my $$n$$ estimates. The standard error is then $$\sqrt(\hat{Var(E)})$$.

On the other hand, the notes here seem to suggest

$$\hat{Var(E)} = \sum_{i = 1}^n (B_i - \bar{B_i})^2\left(\frac{n-1}{n}\right)$$

What am I missing here? And which formula should I use? Many thanks in advance!

Extra question: if the answer is indeed the second formula, will standard errors indeed get smaller (as one would expect) as $$n$$ gets large? I ask because $$\sum_{i = 1}^n (B_i - \bar{B_i})^2/n \approx Var(B_i)$$ for large $$n$$; so it appears that the second estimate is roughly $$Var(B_i)(n-1)$$. It is not clear to me that this is decreasing in $$n$$ (even if $$Var(B_i)$$ goes to zero as $$n$$ gets large).

Consider a statistic of interest $$\theta$$ with a consistent but biased estimate $$\hat \theta$$. The Jacknife estimates $$\hat \theta_{(i)}$$ with is the same statistic, based on the sample that is obtained from removing observation $$i$$. Doing this for all $$i$$ and taking the mean over the values $$\hat \theta_{(i)}$$ gives: $$\hat \theta_{(.)} = \frac{1}{n} \sum_{i = 1}^n \hat \theta_{(i)}$$ As stated above, the estimator $$\hat \theta$$ is biased, so let: $$\mathbb{E}(\hat \theta) = \theta + \frac{b}{n} + O\left(\frac{1}{n^2}\right)$$ Where $$b$$ is the first order bias of the estimator $$\hat \theta$$. Then for the Jacknife estimates, we have a similar expression: $$\mathbb{E}(\hat \theta_{(i)}) = \theta + \frac{b}{n-1} + O\left(\frac{1}{n^2}\right).$$ So, ignoring the higher order terms, we get: $$\mathbb{E}(\hat \theta_{(i)}) - \frac{b}{n-1} = \mathbb{E}(\hat \theta) - \frac{b}{n},\\ \to (n-1)\mathbb{E}(\hat \theta_{(i)} - \hat \theta) = \frac{b}{n}$$ This shows that: $$(n-1) (\hat \theta_{(i)} - \hat \theta)$$ is an `unbiased' estimator for $$\dfrac{b}{n}$$ (at least when we are ignoring all other higher order bias terms). Then using this correction, we have the following 'bias-corrected estimate': $$pv_{(i)} = \hat \theta + (n-1)(\hat \theta - \hat \theta_{(i)}) = n \hat \theta + (n-1) \hat \theta_{(i)}.$$ This is called the pseudovalue.
Now, of course it would be stupid to only use $$pv_{(i)}$$ for one particular value of $$i$$ . So a better bias-corrected estimate is the one that averages over all these, the jacknife estimator: $$\hat \theta_{jack} = \frac{1}{n}\sum_{i = }^n pv_i = n \hat \theta + (n-1) \hat \theta_{(.)}.$$ The pseudovalues $$pv_{(i)}$$ are not necessarily i.i.d. but assume they are anyway. Then, their variance is given by: \begin{align*} {\rm var}(pv_i) &= \frac{1}{n-1} \sum_{i = 1}^n (pv_i - \hat \theta_{jack})^2,\\ &= \frac{1}{n-1} \sum_{i = 1}^n (n \hat \theta - (n-1)\hat \theta_{(.)})^2,\\ &= \frac{1}{n-1} \sum_{i = 1}^n (n \hat \theta + (n-1) \hat \theta_{(i)} - n \hat \theta - (n-1) \hat \theta_{(.)} )^2,\\ &= \frac{(n-1)^2}{n-1} \sum_{i = 1}^n (\hat \theta_{(i)}- \hat \theta_{(.)})^2,\\ &= (n-1) \sum_{i = 1}^n (\hat \theta_{(i)} - \hat \theta_{(.)})^2 \end{align*} Assuming a central limit theorem holds for $$\hat \theta_{jack}$$, then: $$\frac{\theta_{jack} - \theta}{s_n} \to^d N(0,1)$$ where $$s_n$$ is the variance of $$pv_i$$ divided by $$n$$. $$s_n^2 = \frac{n-1}{n} \sum_{i = 1}^n \left(\hat \theta_{(i)} - \hat \theta_{(.)}\right)^2.$$ This 'variance', $$s^2_n$$, is the estimator that you give in your question.
• Thanks for this, though I have 2 questions. 1. When you say things like "biased estimator $\hat{\theta}$", I guess you mean "possibly biased estimator $\hat{\theta}$ "? One can use the jackknife even if one's estimator is unbiased (right?) 2. Shouldn't your pseudo-value be $pv_{(i)} = \hat \theta - (n-1)(\hat \theta - \hat \theta_{(i)})$ (minus not plus)? May 25, 2021 at 9:09
• @afreelunch as far as I know, the jacknife is mainly used as a bias correction (but probably you can also use it if the estimator is unbiased). I think the pseudovalues are correct. you compute $\hat \theta - b/n \approx \hat \theta - (n-1)(\hat \theta_{(i)} - \hat \theta)$ which equals $\hat \theta + (n-1)(\hat \theta - \hat \theta_{(i)})$ which in turn equals $n \hat \theta - (n-1) \hat \theta_{(i)}$ as on slide 7 here.
• Apologies I misread your equation (didn't see you had interchanged $\hat {\theta}$ and $\hat {\theta}_{(i)}$)! May 25, 2021 at 10:55