In each pairs formation period, we screen out all stocks from the CRSP daily files that have one or more days with no trade. This serves to identify relatively liquid stocks as well as to facilitate pairs formation. Next, we construct a cumulative total returns index for each stock over the formation period. We then choose a matching partner for each stock by finding the security that minimizes the sum of squared deviations between the two normalized price series. Pairs are thus formed by exhaustive matching in normalized daily ‘‘price’’ space, where price includes reinvested dividends.
We use this approach because it best approximates the description of how traders themselves choose pairs. Interviews with pairs traders suggest that they try to find two stocks whose prices ‘‘move together.’’ In addition to ‘‘unrestricted’’ pairs, we will also present results by sector, where we restrict both stocks to belong to the same broad industry categories defined by Standard and Poors: Utilities, Transportation, Financial, and Industrials. Each stock is assigned to one of these four groups, based on the stock’s SIC code. The minimum-distance criterion is then used to match stocks within each of the groups.