http://www.bloomberg.com/apps/news?pid=20603037&sid=ak.1o0dVxWHo Sept. 8 (Bloomberg) -- The U.S. stocks rally is âmaturingâ and the risk of a 20 percent retreat in the Standard & Poorâs 500 Index increased after sellers became more aggressive than buyers, Bank of America said. Mary Ann Bartels, an analyst at Bank of America, said her volume intensity model, used to capture the marketâs money flows, showed investor buying peaked last week and selling gained strength. Thatâs signaling the 50 percent rally in the S&P 500 from a 12-year low on March 9 may be near an end, she said. Technical analysts study chart patterns to predict prices. Selling pressure was the third of five indicators watched by Bartels to signal increased probability of a 15 percent to 20 percent âcorrectionâ in the benchmark for American equity, she said. Two others were triggered last month when Chinaâs Shanghai Composite Index began a 16 percent slump and Investors Intelligence said bearish sentiment among newsletter writers shrank to less than 20 percent. âThese are all signs of a maturing rally,â Bartels, who ranked second among analysts who study price charts in Institutional Investor magazineâs most recent survey, wrote in a note published today. âThe risk of a deeper correction is increasing.â The two indicators that continue to support the equities rally are technology share outperformance and an increasing number of stocks trading above their 200-day average price, Bartels wrote. A measure of technology shares in the S&P 500 has jumped 39 percent this year, more than the 13 percent gain for the broader index. The percentage of companies listed on the New York Stock Exchange trading above their 200-day moving average rose to 91 percent in August, the highest in five years. âThe S&P 500 remains in an uptrend, but the data challenges how long this can be maintained,â Bartels said.