  In non-central relativistic heavy ion collisions, the created matter
possesses a large initial orbital angular momentum. Particles produced in the
collisions could be polarized globally in the direction of the orbital angular
momentum due to spin-orbit coupling. Recently, the STAR experiment has
presented polarization signals for $\Lambda$ hyperons and possible spin
alignment signals for $\phi$ mesons. Here we discuss the effects of finite
coverage on these observables. The results from a multi-phase transport and a
toy model both indicate that a pseudorapidity coverage narrower than $|\eta|<
\sim 1$ will generate a larger value for the extracted $\phi$-meson $\rho_{00}$
parameter; thus a finite coverage can lead to an artificial deviation of
$\rho_{00}$ from 1/3. We also show that a finite $\eta$ and $p_T$ coverage
affect the extracted $p_H$ parameter for $\Lambda$ hyperons when the real $p_H$
value is non-zero. Therefore proper corrections are necessary to reliably
quantify the global polarization with experimental observables.
