  [Background:] Software effort prediction methods and models typically assume
positive correlation between software product complexity and development
effort. However, conflicting observations, i.e. negative correlation between
product complexity and actual effort, have been witnessed from our experience
with the COCOMO81 dataset. [Aim:] Given our doubt about whether the observed
phenomenon is a coincidence, this study tries to investigate if an increase in
product complexity can result in the abovementioned counter-intuitive trend in
software development projects. [Method:] A modified association rule mining
approach is applied to the transformed COCOMO81 dataset. To reduce noise of
analysis, this approach uses a constant antecedent (Complexity increases while
Effort decreases) to mine potential consequents with pruning. [Results:] The
experiment has respectively mined four, five, and seven association rules from
the general, embedded, and organic projects data. The consequents of the mined
rules suggested two main aspects, namely human capability and product scale, to
be particularly concerned in this study. [Conclusions:] The negative
correlation between complexity and effort is not a coincidence under particular
conditions. In a software project, interactions between product complexity and
other factors, such as Programmer Capability and Analyst Capability, can
inevitably play a "friction" role in weakening the practical influences of
product complexity on actual development effort.
