Pop quiz: How many people watched the fourth-season premiere of ABC’s Modern Family: 10.7 million viewers, 14.4 million, or 18.8 million? Answer: All of the above. With DVRs in 46 percent of U.S. homes, Nielsen is struggling to keep pace with changing viewer habits by releasing progressively higher sets of numbers for each show as time passes and more fans catch up. (And no, the numbers still don’t include online, on demand, and streaming.) So despite Americans using high-tech innovations to watch record amounts of TV, simply counting viewers is more difficult than ever. Forget Hulu and TiVo, what we really need is somebody with a pocket calculator.