Yards Per First Down Attempt measures the yards a team generates on average on their first downs. Gaining at least some yards on first down opens the door for your next two plays, allowing an offense to mix up running and passing plays.
More about Yards Per First Down Attempt
According to Dr. Massey, an assistant professor at the Yale School of Management, in a Wall Street Journal article he indicated that "When it comes to first down, he says, the magic number is four. That's the number of yards Mr. Massey says teams need to gain on first and 10. Those that do, he says, are more likely to be successful in making a first down and keeping the drive alive".