For decades, analysts had a hard time putting a number on offensive line performance. Other than using the eye test by watching game film, the one single number that was often used was the total number of sacks or the percentage of all pass plays that end in a sack.
In other words, offensive lines were held fully responsible for their quarterback getting sacked. This led to misunderstandings when evaluating historical passers, such as Joe Namath, who spent the majority of his career with the New York Jets.