When the Dow Jones industrial average plummeted 508 points in one unnerving day last week, losing 22.6 percent of its value, it marked the widely followed indicator's largest-ever percentage loss - notwithstanding initial statistics from various news services that indicated otherwise.

The tumble without a doubt represented the biggest point loss ever in the Dow average, but a larger percentage loss appeared in a table and article in The New York Times reporting on the 10 largest percentage declines in the average. For Dec. 12, 1914, a decline of 24 percent was cited, with the average reportedly having fallen 17.42 points, to close at 54.

Sorting out why that day did not represent the biggest loss in percentage terms sheds light on some interesting facts of Dow history.

The change for that day was calculated on the previous close, but that close had taken place nearly six months before. The stock exchange was shut in July 1914 as World War I began, and it did not reopen until Dec. 12 that year. The close on July 30, 1914, for the Dow, which at that time consisted of 12 industrial stocks, was 71.42; when the market opened its doors again on Dec. 12, the average advanced to 74.56, a gain of 4.4 percent.

So why the decline noted in recent tables?

Two years later, in September 1916, a new list of 20 stocks was adopted for the Dow, and the average was recomputed back to Dec. 12, 1914. As recalculated, the close on Dec. 12 was 54.62. Although placing that close against the July close of the old average is an apples-and-oranges comparison, doing so yields a decline of nearly 24 percent - and that is what led to the erroneous report that Dec. 12, 1914, was the worst day in stock market history.

Since 1916, the Dow Jones average has been changed numerous times, with various stocks replacing issues removed from the index, and in 1928 it was expanded to 30 stocks, from 20.The method of computation for the average, however, was revised each time there was a change, so the figures since the recalculation back to 1914 are continuous.