Measurement forces us to confront decisions we don't wish to make.

From The effects of the medical marijuana market on substance abuse by Tyler Cowen.

The argument is about the possible consequences of drug legalization, looking at medical marijuana as a particular case. What happens to drug use when marijuana for medical purposes is legalized? From the abstract of the original study Cowen is discussing:

I find that growth in the legal medical marijuana market significantly increases recreational use among all age groups. Increased consumption among older adults has positive consequences in the form of an 11% reduction in alcohol- and opioid-poisoning deaths. However, increased consumption among youths leads to negative externalities. Raising the share of adults registered as medical marijuana patients by one percentage point increases the prevalence of recent marijuana use among adolescents and young adults by 5-6% and generates negative externalities in the form of increased traffic fatalities (7%) and alcohol poisoning deaths (4%).

With complex, dynamic multi-causal systems, outcomes are hard to forecast, particularly across disparate populations. Cowen sees this as evidence supporting his assumption that:

When it comes to “those who already are screwed up,” namely the older generation, it is best to shunt them off into pot, compared to the relevant alternatives. But when it comes to the younger generation, the new norm that “pot is OK” may in fact not be best in the longer run. So in sum,while I (TC, not the author necessarily) favor marijuana decriminalization, we should hold mixed moods towards its practical effects.

Well, not just mixed moods. If we accept at face value the outcome of this study, we are explicitly sacrificing the young to improve the circumstances of the older generations. Whose lives do we value and to what extent? Measurement forces us to confront decisions we don't wish to make.