PERUSING the latest Harper's Index, that quirkily informative monitor of life in America, we find the following statistics: Number of plastic liners used by Florida prisons last year to combat chafing caused by leg irons: 280. Rank of Oprah Winfrey among the celebrities Americans believe to be most qualified for the US presidency: 1. Chances that the phrase you will appears on a page of Bill Gates' new book, The Road Ahead: 1 in 5. Number of times Ringo Starr's Private Issue credit-card ad ran during the six-hour Beatles documentary: 6. And finally, the chances that an ad in the January issue of Esquire offers products or services relating to hair-loss: 1 in 4.