How to use the Harris Index Match-Up Tool
The Harris Index is an opponent-independent tool that measures how far above or below average a team has held an opponent over the past five weeks. A large positive number means that Team X's opponents have exceeded their five-week moving average against Team X, and thus Team X represents an attractive match-up. A large negative number means the opposite: be wary of using your player against his opponent this week.
Example: If the Seattle Seahawks rate a "minus-6" against RBs, that means they've held opposing RBs to six fantasy points below those RBs' five-week moving average. Thus you can expect that this week's opposing RB will be held below his typical production.
Any defense whose performance is outside two standard deviations gets a dark color coding: red for a bad match-up, green for a good one. Any defense whose performance is outside of one standard deviation gets a light color coding. And any defense that is within one standard deviation can't be considered a statistically significant negative or positive match-up, and isn't shaded.