# Convergence of $\hat \beta_N - \hat \beta_{N-1}$ where $\hat \beta_N$ is the least squares solution of $Y_N = X_N\beta_N + \varepsilon_N$?

Suppose we are given a set of random observations $$\{y_i,x_{i1},\dots,x_{ip}\}_{i=1}^N$$. Based on these observations, we can form the multiple linear regression model in matrix form $$Y_N = X_N\beta + \varepsilon_N,$$ where the error, conditioned on $$X$$, is normally distributed with mean zero and finite variance. I have used the subscript $$N$$ to explicitly indicate that we have used $$N$$ observations.

When we solve this model with least squares we find the estimated regression coefficient vector $$\hat \beta_N$$. We could also leave out the last observation and solve the model $$Y_{N-1} = X_{N-1}\beta + \varepsilon_{N-1},$$ to find the estimated regression coefficients $$\hat \beta_{N-1}$$.

The explicit expression for the estimated regression coefficients is well-known. We have \begin{align} \hat \beta_N &= \beta_N + (X_N^TX_N)^{-1}X^T \varepsilon_N, \\ \hat \beta_{N-1} &= \beta_{N-1} + (X_{N-1}^TX_{N-1})^{-1}X^T \varepsilon_{N-1}. \end{align} As $$N$$ gets large intuitively it seems there will be negligible difference between the estmiated regression coefficients $$\hat \beta_N$$ ad $$\hat \beta_{N-1}$$. But can we quantify this precisely? Is it possible to show that $$E(N) := \hat \beta_N - \hat \beta_{N-1}$$ converges to zero in probability or almost surely as $$N \to 0$$? And in particular, how fast it converges to zero?

• I will comment also on the convergence in $L^p$ (your question on mathoverflow), if you edit the question accordingly. Dec 14 '20 at 16:59

(From the discussion on MO) I guess that the subscript $$N$$ in $$\beta_N$$ is a lapse.
It is well know that, conditionally on $$X_N$$, $$\hat \beta_N$$ is unbiased and $$\mathrm{Cov}(\hat \beta_N) = \sigma^2 \big(X_N^T X_N\big)^{-1}$$.
Now in order to have some convergence, one naturally needs additional (to the usual Gauss–Markov) assumptions. Say, if $$x_i$$ are iid and square integrable, then $$X_N^T X_N = \sum_{i=1}^N |x_i|^2 \sim n\,\mathrm{E}[|x_1|^2], n\to\infty,$$ almost surely. As a result, $$\hat \beta_n \overset{\mathrm P}\longrightarrow \beta$$, $$n\to\infty$$, moreover, $$\sqrt{n} |\hat\beta_n - \beta|$$ is bounded in probability. (So the convergence rate is loosely $$1/\sqrt{n}$$.)
• Thanks for addressing this question. My central issue at the moment is explained in the comments of my latest post on stats.stackstackexhange. Essentially I need to know the order of magnitude of the moments $E[(\hat \beta - \beta)^k]$ for $k \ge 3$, as $n \to \infty$ because I am doing a second order Taylor expansion and I want to be sure it is safe to drop the third and higher order terms. Dec 14 '20 at 17:17