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节选自一篇英文文章，因为相关知识不懂，所以实在难以看懂，请高手帮忙看看是个什么意思，
Kothari, Shanken, and Sloan (1995) argue that survivor biases in Compustat
data may partially explain the relation between book-to-market ratios and
security returns. They argue that there are two sources of bias. First, prior to
1978, Compustat routinely included historical financial information of firms. Second,
Compustat may back-fill the financial information of firms that delayed the
reporting of their financial statements for reasons related to financial distress. The
problem with this type of back-filling is that the firms that emerge from financial
distress are more likely to be back-filled. However, the accumulating evidence
suggests that there is a positive relation between book-to-market ratios and security
returns (e.g., Davis, 1994; Chan, Jegadeesh, and Lakonishok, 1995; Barber
and Lyon, 1996b). Furthermore, Chan, Jegadeesh, and Lakonishok (1995) argue
that the survivor bias in Compustat data is small.
In this research, we are forced to either ignore the possible relation between
book-to-market ratios and security returns or use data that we know are subject
to some survivor bias (though the extent of the bias is contested). We choose
to include book-to-market ratios in our analysis for four reasons. First, in eventstudies over long horizons, the survivor bias will lead to biases in results only if
sample firms are more or less likely to have been back-filled by Compustat than
the general population. A survivor bias in Compustat data is not sufficient to reject
results that document significant long-horizon abnormal returns. Second, the bookto-
market and size/book-to-market reference portfolio and control firm approaches
should control well for the survivor biases in Compustat data. If book-to-market
ratios are an instrument for survivor bias in Compustat data, we can control for
the survivor bias inherent in Compustat data by matching sample firms to firms of
similar book-to-market ratios. Third, we have reestimated all of our results in the
1979 through 1994 subperiod. Kothari, Shanken, and Sloan (1995) indicate that
Compustat did not include historical financial information for firms in its database
during this period, though the survivor bias from delayed financial reports persists.
The general tenor of our results is similar during this subperiod. Fourth, we have
reestimated our results by drawing samples from the population of firms described
in Table 2, but without regard to the availability of book-to-market ratios. The
results that employ size decile portfolios, size-matched control firms, the Fama
French three-factor model, and the equally weighted market index are similar
to those that we report later. Barber and Lyon (1996a) thoroughly discuss the
impact of dropping the requirements for size and book-to-market data.
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