Integrating the two steps of an arbitrage pricing theory (APT) model l
eads to a reduced-rank regression (RRR) model. So the results on RRR c
an be used to estimate APT models, making estimation very simple. We g
ive a succinct derivation of estimation of RRR, derive the asymptotic
variance of RRR estimators for a general case, and discuss how undersi
zed samples (more assets than time periods) can be dealt with.