In part, that may be because the work is just so complicated. To come up with estimates, the group had to link several large databases of information. The NHANES survey results can tell you what a broad spectrum of American plates look like on any given day, though it's worth noting that NHANES looks only at the most recent 24-hours' worth of food, so not everyone in this study who said they ate a lot of beef necessarily eats that much all the time (still, it's by far the best data available on what the U.S. eats, and NHANES does look at whether those 24-hour food recalls are representative of a person's overall diet, so you're not off the hook!). But that doesn't tell you anything about the emissions caused by those foods. To do that, you have to go to the Food Commodities Intake Database, run by the EPA, and figure out how much meat might be in that meat lasagna, or how many tomatoes are in a generic salad. From there, you have to link the quantities of each type of food to the emissions associated with producing it.