Coaches and players preach it.We've got to get off the field on third down.The Bucs? They've been spending so much time on the field they could tell you all you ever wanted to know about the grass at Raymond James Stadium.Tifway 419 Bermuda. Dense. Tough. Weed-resistant.Last season, the defense allowed opponents to convert only one out of every three third downs. This season, it's allowing opponents to convert half of them. What has changed in a year?Not much.The third-down defense you saw last season was an illusion.We know what the numbers say. We've double- and triple-checked them. They're accurate.They don't, however, give us a complete picture.Third-down conversion percentage is not a reliable indicator of the quality of a defense.That's because defenses face relatively few third downs. The Bucs defended more than 1,000 plays last season. Less than 20 percent were third downs. In fact, they were a couple of plays away from facing the fewest third downs.By emphasizing third down, we risk overlooking performance on early downs. Here's why that's important: Defenses that perform poorly on first and second down but well on third down tend to decline the next season, Football Outsiders research shows.That's exactly what has happened to the Bucs.The defense was bad on early downs last season. It allowed the third-most yards per play on first down. While it was a little better on second down in terms of yards allowed, it allowed opponents to gain a first down 35.8 percent of the time, the fourth-highest rate.So as sparkling as the third-down percentage was last season, it wasn't especially meaningful. What good is a low percentage if opponents are getting first downs before they get to third down?Compounding the Bucs' struggles this season is their inability to force opponents into predictable situations. Instead of watching for passes, screens and draws on third down, they're having to worry about runs and play-action, too."The better you are on first and second down, then the better chance you have on third down," safety Keith Tandy said."Obviously if you get them in more defense-favorable situations on third down, then you should have more success."Case in point: On Sunday against Carolina, Tampa Bay bit on a run fake and left tight end Ed Dickson wide open on a third and 1. Dickson gained 14 yards to extend a drive that eventually ended with a touchdown.The chances of a turnaround this season are slim. The Bucs are allowing 5.4 yards per play on first and second downs. That's around league average, but it's only a slight improvement over last season.Until the defense is more consistent on all downs, it's going to see more of the field than the grounds keepers.Contact Thomas Bassinger at email@example.com. Follow @tometrics.