The Fritsch-Waugh-Lovell theorem: if<p>y = a<i>x + b</i>y + c<i>z + (zero-mean nuisance),<p>I can estimate a least-squares model of the form<p>y&#x27; = a</i>x + b*y + (zero-mean nuisance)<p>and get a correct estimate of b, as long as y and z are uncorrelated.<p>(Where this breaks down: for example, a regression of IQ on age and arm length; omit age and you&#x27;ll get longer arms (older kids) producing higher IQ scores, because age and arm length are correlated.)