In what setup would the difference between the two be measurable?
I dunno. Not an astronomer. But there are lots of different strategies for measuring things, which come with their own particular strengths and weaknesses, so I wouldn't be surprised if some available measures of some fraction had different inherent bounds or precisions based on available data.
(For example, in genomics, it's not uncommon to have a lower bound with a confidence interval; in fact, every GCTA study using SNPs produces a lower bound with a somewhat loose confidence interval, and this has tripped up some commentators who, upon observing an esti...
On the abundance of extraterrestrial life after the Kepler mission Amri Wandel
Some recent calculation of the Drake Equation with estimates of the likelihood and logevitiy of civilizations
Related: The Great Filter and Planets in the habitable zone, the Drake Equation, and the Great Filter