# Correlated multivariate linear least squares

Linear Least Squares solves $y = X\beta + \varepsilon$ for $\beta$ when $y, \beta$ are vectors of size $n$. and $\operatorname{Var}[\,\varepsilon \mid X\,] = \sigma^2 I_n$ (spherical errors). I have multivariate measurements, meaning that $y$ is a matrix, but also, the different variates of each measurement are correlated and I have a known covariance matrix for each measurement (also means that each measurement have a different covariance). So, obviously, the spherical errors assumption of ordinary least squares doesn't hold. Is there some extension for least squares that allows to solve such problem?

clarification update: In my problem, every row of $y$, $y_i$, is a column vector $\begin{pmatrix}y_{i1},y_{i2},y_{i3}\end{pmatrix}$ which is a vector of random variables, with a known (not diagonal) covariance. That's a 3d location measurement for input time $X_i$. (every row in $X$ is 1,time, and time squared).

• What do you mean with multivariate measurements? And what are variates? Could you describe your setting more formally, like you did in the Simple linear regression setting? Feb 23, 2016 at 12:50
• added clarification Feb 23, 2016 at 13:11

Instead of $y$ being a matrix as in General Linear Model, I look at every measurement independently, so it's a column vector. so, as in Generalized Linear Squares we have: $$\hat\beta = \underset{b}{\rm arg\,min}\,(y_i-X_ib)^t\,P^{-1}_i(y_i-X_ib)$$ for the $i$ measurement, where $P_i$ is the covariance matrix. But we want to minimize across all the measurements. We should use the sum for that (because we try to minimize sum of squares), so our problem is to find: $$\hat\beta = \underset{b}{\rm arg\,min}\,\sum_i(y_i-X_ib)^t\,P^{-1}_i(y_i-X_ib)$$ If we follow the proof to Generalized Least Squares, which gives the optimal solution: $$\hat\beta = (X^tP^{-1}X)^{-1} X^tP^{-1}y$$ it's easy to see that with the sum we'll get (it's just sums of matrix derivatives): $$\hat\beta = (\sum_iX_i^tP_i^{-1}X_i)^{-1} \sum_iX_i^tP_i^{-1}y_i$$
• I am facing a similar problem. I want to use the Generalized Least Squares for estimating the parameters of a model given some data. My problem is that I start with uncorrelated data $x_i$ but before the fit, I need to apply the following transformation to the data $y_i = x_{i-1}-2x_i+x_{i+1}$ and there is when my data (and their errors) get correlated. So I am training to compute the covariance matrix $P$ that you talk about in your post. But I am not sure how to use the aforementioned transformation formula to build it. Could you have any advice? May 20, 2018 at 15:58