Nielsen Media Research has conceded making an error and is performing a recount after the company's ratings on Tuesday initially indicated that ABC's "World News" most likely had its smallest audience ever. The dispute with Nielsen was a downer for the network after the entertainment division—keyed by the National Basketball Association finals—won in the prime-time ratings for the first week since September. It was ABC's most-watched week in the summer in five years, according to Nielsen. ABC asked Nielsen for an investigation after its ratings showed "World News" averaged 6.2 million viewers last week (4.3 rating, 9 share), its worst...