Most fianancial data sources eliminate data on stocks that are no longer trading. This creates an upward bias to results when performing simulations, especially with shorting strategies.
However, data is now available from Tradestation on stocks no longer trading. This will tend to eliminate the upward bias resulting from most simulations run on data sources that eliminate stocks no longer trading:
"Data on stock symbols that no longer trade is now available. Now that we have opened up access to history on all stocks, including those stocks that no longer trade, you now have the ability to test strategies on a larger number of stocks. This may be a more valid way to perform strategy testing since strategy tests can be run both on companies that currently trade as well as companies that no longer trade, thereby helping to eliminate a potential upward bias that may result from running tests on only companies that currently trade."