Piper Jaffray downgraded 99 Cents Only Stores (NDN

) to underperform from market perform, explaining that the companies shares have been up about 22% since the beginning of the year.

Analyst Mitchell Kaiser says prices are higher on expectations of improved economic conditions for lower-income consumers, among other things. He does not see such catalysts for 99 Cents Only Stores shares happening. He says 1.8% March same-store sales comparables are slightly below his 2.0% estimate. He notes that since 1999, 99 Cents Only Stores has experienced significant operating margin erosion due to higher Selling General & Administrative expenses. He does not believe 99 Cents Only has a clear plan for turning around. He keeps a $10 stock price target.