Adding to the problem of comparing......
THC concentrations over time are the varying conditions under which the plants are grown and how that has changed over time.
Lumping together outdoor and indoor confiscations, just produces fruit salad.
Comparing indoor crops over 20-30 year periods presumes the technology for indoor growing has remained constant. In other words, it may not be that contemporary cannabis is stronger so much as the techniques for growing it have improved significantly. It may well be, that a 1970's seed grown indoors under today's state of the art technologies would result in a THC concentration not significantly different from what would be achieved by growing a 2016 seed under the same conditions.
At the same time, restricting the analysis to the potency of outdoor crops only encounters a similar problem. Namely, were the environmental conditions the same in both years? Was the amount of sun, rain, temperature, etc. similar?
I can say based on my experience that it's not stronger so much as it is CONSISTENTLY stronger. There was some great herb floating around North Texas in the late 60's and early 70's, but it was rare. Now, that quality is seems much closer to the standard.
JMHO
In a time of universal deceit - telling the truth is a revolutionary act. - George Orwell
