  We address the statistical and optimization impacts of the classical sketch
and Hessian sketch used to approximately solve the Matrix Ridge Regression
(MRR) problem. Prior research has quantified the effects of classical sketch on
the strictly simpler least squares regression (LSR) problem. We establish that
classical sketch has a similar effect upon the optimization properties of MRR
as it does on those of LSR: namely, it recovers nearly optimal solutions. By
contrast, Hessian sketch does not have this guarantee, instead, the
approximation error is governed by a subtle interplay between the "mass" in the
responses and the optimal objective value.
  For both types of approximation, the regularization in the sketched MRR
problem results in significantly different statistical properties from those of
the sketched LSR problem. In particular, there is a bias-variance trade-off in
sketched MRR that is not present in sketched LSR. We provide upper and lower
bounds on the bias and variance of sketched MRR, these bounds show that
classical sketch significantly increases the variance, while Hessian sketch
significantly increases the bias. Empirically, sketched MRR solutions can have
risks that are higher by an order-of-magnitude than those of the optimal MRR
solutions.
  We establish theoretically and empirically that model averaging greatly
decreases the gap between the risks of the true and sketched solutions to the
MRR problem. Thus, in parallel or distributed settings, sketching combined with
model averaging is a powerful technique that quickly obtains near-optimal
solutions to the MRR problem while greatly mitigating the increased statistical
risk incurred by sketching.
