Survivorship and Exchange Bias

One problem that all other databases of US Stocks suffer from are the survivorship bias and the exchange bias.

The survivorship bias occurs when a database only includes currently-listed companies and ignores delisted companies. Each year, hundreds of companies delist because of bankruptcy, merger, or acquisition. Half the companies that were in the S&P 500 ten years ago are no longer part of it.

The exchange bias occurs because many companies move either from the over-the-counter market onto an exchange, or from one exchange to another. Ignoring the stock history of a company before it moved to the NYSE means that the analyst has an incomplete and inaccurate picture of the company’s history.

Companies such as Exxon, Mobil, Disney, Wal-Mart and others traded for years and even decades before listing on the NYSE. Most banks and insurance companies traded over-the-counter before the 1970s meaning that by ignoring those years, their stock histories are incomplete.

Any database that excludes either companies that have delisted or companies that listed elsewhere before moving onto the NYSE provide an incomplete and inaccurate picture of the stock market in the past.

Global Financial Data is the only company that has made an effort to eliminate the survivorship and exchange bias that is inherent in other databases by including not only delisted companies, but companies that traded over-the-counter or on regional exchanges.

Only a subscription to Global Financial Data will provide the complete share histories that are needed to avoid biased results that other databases provide.