Error on "Double Pairs" example page?

[Doug]

http://www.sudokuoftheday.com/pages/techniques-4.php

I'm just trying to learn this game, but it looks to me like the top center box can contain four 2's, not just two. The bottom two empty cells, to the right of the 9, show only 3,5,7 pencilmarks. To make this example work, shouldn't there be a hard 2 somewhere on the third row? If not, can someone please tell me why there can't be 2's in those cells?

I can't get past this example without getting hung up on this.

[jstorm1031]

Hey Doug -

I can see your confusion. The answer is that there is a Candidate Line Test in Box 1. The only place you can put 2's in Box 1 are in the third row. Therefore, you can safely rule them out of the third row in Box 2. I hope that makes sense.

Jim

[Doug]

OK, so the two 2's in the lower left two cells in the first box say it's 2 is in that row, in that box, so it can't be elsewhere.

[jstorm1031]

Yes - this is a Candidate Line Test, and is a very common method of ruling out numbers from other boxes. It would appear to be about the 2nd most common hint given, after Single Position Tests. I typically will go through and look for any SPT and CLT before I do anything else, because on Medium level or less, this is sometimes all you need to do to solve the problem.

jstorm