  Molecular emission is used to investigate both the physical and chemical
properties of protoplanetary disks. Therefore, to accurately derive disk
properties, we need a thorough understanding of the behavior of the molecular
probes we rely on. Here we investigate how the molecular line emission of
N$_2$H$^+$, HCO$^+$, HCN, and C$^{18}$O compare to other measured quantities in
a set of 20 protoplanetary disks. Overall, we find positive correlations
between multiple line fluxes and the disk dust mass and radius. We also
generally find strong positive correlations between the line fluxes of
different molecular species. However, some disks do show noticeable differences
in the relative fluxes of N$_2$H$^+$, HCO$^+$, HCN, and C$^{18}$O. These
differences occur even within a single star-forming region. This results in a
potentially large range of different disk masses and chemical compositions for
systems of similar age and birth environment. While we make preliminary
comparisons of molecular fluxes across different star-forming regions, more
complete and uniform samples are needed in the future to search for trends with
birth environment or age.
