How computers helped stampede the stock market: Many analysts blame automated program trading for the worldwide plunge — But the machines only did what they were told

Two trading days before Oct. 19 — the stock market´s Black Monday — analysts were already flying warning flags. Program trading, they said, had contributed to the 166-point total drop in the Dow Jones Industrial Average over Thursday and Friday, Oct. 15 and 16. During Monday´s 508.32-point plunge, some program — or computerized — trading added to the panic as well.