All text is ephemeral. Some texts are more ephemeral than others. The web has proved to be among the most ephemeral and changing of information vehicles. The research note revisits Koehler's original data set after about 20 years since it was first collected. By late 2013, the number of URLs responding to a query had fallen to 1.6% of the original sample. A query of the 6 remaining URLs in F ebruary 2015 showed only 2 still responding.
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Oguz et al. (2015) studied this question.
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