BOSTON ( TheStreet) -- Stocks have been on a remarkable ascent since the "world is ending" bottom in March 2009. (The S&P 500 Index is up about 80%.)Investors have become more optimistic and willing to assume higher risk. However, many are still waiting for signs of economic stability, what with elevated unemployment, the down-and-out housing market and woes in Europe and Japan. Today's news that Standard & Poor's put U.S. sovereign debt's AAA rating, the highest, on a "negative" outlook is hammering stocks, with the S&P 500 down almost 2%. Since the financial meltdown and deep economic recession of the past three years, investors have been skittish about pouring money into stocks. Today's S&P release shows that every bit of negative news has the potential to derail the stock-market rally. Digging deeper, though, reveals that U.S. equities are attractively valued. While a 10% drop in the market is certainly a possibility, if you're waiting for a double-dip or a retracement to early 2009 levels, odds are that you might be waiting for a long time. TheStreet Ratings' quantitative model -- overseen by a unit of TheStreet -- currently rates 37% of our 6,000 stock universe a "buy," the highest since September 2007 (see graph below). Keep in mind, the model focuses on both risk and reward, looking at not only the fundamentals of a particular stock (such as growth in revenue and cash flow) but also the overall financial strength and possibility of bankruptcy. This leads to a favoritism toward stocks with solid growth, clean balance sheets, strong past relative performance and attractive valuations.