That's a very important point. The results can't really be mapped to a one-dimensional score of some kind of nootropyness. Especially with higher doses, many of the nootropics have wildly different effects.

If one makes you giddy and bursting with creative ideas and another gives you drone-like immunity to distractions, they can't be linearly converted to equivalent doses of caffeine.

I think the survey's score chart is best understood as a "how much do you like it" ranking instead of a conversion chart.