The phrase "better measurement" means improving the way something is calculated or assessed. It involves finding more accurate or reliable ways to quantify and understand a particular thing or concept. Full definition
But we still don't know what it is, and we also haven't made very good measurements of it. (newscenter.lbl.gov)
To avoid this, scientists measure oxytocin in the blood, but they have been unsure how well these measurements reflected the hormone's levels in the central nervous system. (sciencedaily.com)