You might be wondering how the potency of marijuana can be compared over a long span of time. Who was doing the measuring "back in the day", and how would we know whether their methodology was reliable? What varieties of marijuana were being measured, and which parts of the plant? Grown under what conditions?
While it's fun to ridicule these studies for being empirically dubious, I would think (and hope) that their conclusion is correct. Growing technology is very advanced. For example, this article describes a very sophisticated regiment for plants grown hydroponically:
It was a relatively small operation: the lights and their installation had cost about fifteen thousand dollars, and power and nutrients had cost an additional twelve thousand or so. The array of nutrients along the walls included specialized growing products such as Bud Blood (“promotes larger, heavier & denser flowers and fruit”) and Rizotonic (a powerful root stimulant). “Voodoo Juice is going to go in here, and Scorpion, and it goes on and on,” the Kid said. Every three or four days, she ran purified water through her hydroponic growing medium for a full day, in order to give the plants a break. After the full, eight-week growth cycle, the Kid planned to harvest her crop and clear out.
Do you think anyone gave a shit about fancy fertilizers back in the 70s? I doubt they even had notions like 'nug', 'schwagg', or 'mid-grade' (whether or not there really is such a thing as mid-grade would make an interesting topic for another post). Of course marijuana is stronger today; there are a wide variety of advanced technologies specifically developed to assist in its growth. Unfortunately, these advancements are cited by the antidrug crowd just to motivate former pot smokers to be hypocritical.
To some extent the whole discussion of relative strengths is moot due to the fact that a lot of people now smoke bud/nugs/"the Good shit" (this is my favorite name--it makes reference to the fact that nug is probably God-given, but again, that's for a different post), whereas--from the information I can gather--people didn't used to smoke bud exclusively, just as they didn't wear seat-belts or refrain from all-day cigarette smoking. Life was so shitty back then that standards barely existed. If you wanted to throw a bunch of people into a mock--but fully functional--prison and randomly assign roles like 'Guard' and 'Inmate', then it was totally cool.
Again, relative strengths is irrelevant because people used to indiscrimately smoke all kinds of the plant--bud or not--whereas today a lot of smokers smoke bud exclusively. They also smoked in different ways. My informant claims that people smoked joints exclusively around 1971-1975. At the time, the standard amount for purchase was an ounce. My informant, a dealer around this time, would wrap the ounces in ribbon for show, as her clientele was made up mostly of women. That is to say, the standard purchase amount for casual buyers was an ounce. Today, the typical purchase amount is an eighth of an ounce. It's true that weed is stronger today than it was "back in the day", but it's also more expensive, and people buy less of it.