Accepting the assumption that a herbarium specimen is randomly gathered (a tenuous proposition), then we can determine if “rare” plants are encountered less often compared to “common” plants (rare plants defined as being on the CNPS list).
In the central Sierra Nevada region centered about Yosemite (Tuolumne, Mariposa, Madera and Mono Conties), with a total of 70,966 herbarium specimens to analyze, the results is that: yes, rare plants are collected less often as compared to common plants.
The difference is significant (using the non-parametric Kolmogorov-Smirnov two-sample test) at p=0.003. Box plot below...
Those rare plants with the but a single specimen are:
The most frequently collected “rare” plants are (a.. >70 total specimens)
Astragalus kentrophyta var. danaus
Perhaps being infrequently collected does not diagnose rarity: there are 464 non-rare species in the Yosemite region which have been collected only once.