Stocks opened lower on Tuesday as ongoing concerns over the strength of the economic recovery looked set to eclipse data expected to show U.S. manufacturing expanded in August.

After a 50-percent run up in the S&P 500 index since early March, analysts said investors were pausing to assess whether the gains are justified by economic fundamentals.

The Dow Jones industrial average <.DJI> dropped 38.39 points, or 0.40 percent, to 9,457.89. The Standard & Poor's 500 Index <.SPX> shed 3.73 points, or 0.37 percent, to 1,016.89. The Nasdaq Composite Index <.IXIC> fell 6.92 points, or 0.34 percent, to 2,002.14.

(Editing by Padraic Cassidy)