The number of companies sued on stock-fraud claims dropped to 169 from 223 in 2008, compared with an annual average of 197 cases over the previous decade, according to the study, released today by Stanford Law School’s Securities Class Action Clearinghouse and Cornerstone Research. Almost half of the decline stems from the lack of new grounds for suits related to credit losses, the researchers found. A drop in stock-market volatility was also a significant reason that the number of stock-fraud suits was relatively low in 2009, the study said.