The problems had spoiled the American Stock Exchange's statistics-keeping for four days in a row and caused it to release incorrect market-value indexes for July 16-18 and July 21. It released corrected index figures for the four days. An employee of SIAC, a computer cooperative that provides data for the Amex and the New York Stock Exchange, said the errors were caused by a software bug that was corrected Monday night. The NYSE was not affected. The correct closing market-value indexes: Wednesday, July 16, 266.01, up 0.65; Thursday, July 17, 266.98, up 0.97; Friday, July 18, 266.97, down 0.01, and Monday, July 21, 266.38, down 0.59.