To demonstrate that a planned commercial will be cost effective, at least 60% of those watching the

Question:

To demonstrate that a planned commercial will be cost effective, at least 60% of those watching the programming need to see the commercial (rather than switching stations or using a digital recorder to skip the commercial). Typically, half of viewers watch the commercials. It is possible to measure the behavior of viewers in 1,000,000 households, but is such a large sample needed?