A criticism of taxing marijuana by weight is that it should incentivize more potent product, while a price-based tax should not. Critics have suggested that customers will pay much more for an ounce of potent product – which would bear the same tax as a weak ounce. Taxing by price, they say, will avoid that incentive.
But new data shows that potency and price are not tightly related.
Dr. Caroline Weber of the University of Washington sent me this chart: https://newtax.files.wordpress.com/2018/11/table_for_pat_oglesby.pdf. It comes from her and from two University of Oregon economists, Ben Hansen and Keaton Miller, and considers about 60 million retail transactions.
If price followed potency directly, if a gram of 10-percent THC cannabis sells for $7, a gram of 15-percent THC cannabis would sell for $10.50 = $7 (15/10).
But in fact, in those 60 million transactions, a shift between 10 and 15 percent potency typically increases the price of a gram by much less than the 50 percent increase in potency.
In Table 1, the regression coefficient of THC concentration for the tax-inclusive price of cannabis in Washington State is 0.188, if I have the terminology right.
Using Table 1: If a gram of 10-percent THC cannabis sells for $7, a gram of 15-percent THC cannabis would sell for $7.94 = $7 + ((15 – 10) 0.188).
Meanwhile, using Table 2: If THC increases by one percentage point, price increases by 2.05 percent. So if a gram of 10-percent THC cannabis sells for $7, a gram of 15-percent THC cannabis would sell for $7.75 = $7 X (1.0205 to the 5th power)).
Now the sophisticated weight-based schemes in place in Alaska, California, Colorado, Maine, and Nevada, all tax multiple categories of product to tax. Potent bud is typically taxed at about three times the rate per ounce used for less potent bud. These weight based taxes may reflect potency better than price-based taxes.
In any event, price is not a very good proxy for potency.