Bugs are usually in associations with other bugs in a software system, e.g., a bug may result from another bug. However, such bug associations are implicit and usually cannot be traced without a significant amount of effort. Intuitively, if a bug association is easier to trace, the involved bugs can be fixed in a cleaner way. However, there is little evidence on the explicitness of bug associations. In this paper, we aim to evaluate the explicitness of bug associations, so as to get a basic understanding on such associations. To this end, we defined a metric to quantify the explicitness of a bug association, and conducted an empirical study on 11 non-trivial Apache open source software systems. The main findings are summarized as follows: (1) From the perspective of code change history, around 29% of bug pairs are not explicitly associated, and about 71% are explicitly associated to some extent; (2) Bugs in the association of Container have relatively strong association explicitness, while bugs in the association of Blocked or Blocker, Cloners, and Dependent have relatively weak association explicitness. These findings provide insights on software analyzability to practitioners and researchers.