The Breakdown:

It turns out joints are smaller (i.e., contain less marijuana) than was thought in prior reports and estimates.
The new estimate is that an average marijuana joint has 0.32 grams of pot inside. Seems a little small right?
The Researchers:
The study was done by Greg Ridgeway and Beau Kilmer.
Greg Ridgeway is an Associate Professor of Criminology and Statistics as well as Director of the M.S. Criminology program at University of Pennsylvania. Ridgeway previously was Director for the RAND Center on Quality Policing as well as the RAND Safety and Justice Program.
Beau Kilmer is the Codirector of the RAND Drug Policy Research Center as well as Senior Policy Researcher and RAND.
The Math:
There's 28.349 grams in an ounce of marijuana .
0.32 / 28.349 grams = 0.011 ounces of marijuana in an average joint according to the new studies.
The U.S. Office of National Drug Control Policy has half a gram set as its baseline.
These new estimates are based on 11 years of drug transactions in over 40 U.S. communities, totaling over 10,000 transactions.
The Takeaways:
This information is important according to Greg Ridgeway because "it turns out to be a critical number in estimating how much marijuana is being consumed..." and much more.
Wiz Khalifa's joints obviously were not taken into account for this study.
We can imagine that this estimate will be used to shape marijuana policy in the United States.
If a joint is now estimated at .32 grams vs. .5 grams, all of the sudden someone who consumes 1 gram per day is smoking an *estimated* 3 joints a day (in the eyes of the law) vs. 1 fat joint per day as many do.
This sort of thinking mixed with these kinds of studies are what lawmakers will use to determine the regulation of marijuana on both the recreational and medical sides of the spectrum.
References & Further Reading Recommendations:
"New Penn Research Shows Average Joint Contains Much Less Marijuana Than Thought" (UPenn)
"Bayesian inference for the distribution of grams of marijuana in a joint" - Greg Ridgeway and Beau Kilmer (Drug and Alcohol Dependence Journal)
Greg Ridgeway Profile - (University of Pennsylvania)
Beau Kilmer Profile - (RAND Corporation)
Unrolled Joint Image Credit - (Wikimedia Commons)