  We report here on the results of two studies using two and four monthly web
crawls respectively from the Common Crawl (CC) initiative between 2014 and
2017, whose initial goal was to provide empirical evidence for the changing
patterns of use of so-called persistent identifiers. This paper focusses on the
tooling needed for dealing with CC data, and the problems we found with it. The
first study is based on over $10^{12}$ URIs from over $5 * 10^9$ pages crawled
in April 2014 and April 2017, the second study adds a further $3 * 10^9$ pages
from the April 2015 and April 2016 crawls. We conclude with suggestions on
specific actions needed to enable studies based on CC to give reliable
longitudinal information.
