  Context: Empirical studies in software engineering are typically centered on
human subjects, ranging from novice to experienced developers. The experience
of these individuals is a key context factor that should be properly
characterized for supporting the design of empirical studies and interpreting
their results. However, the criteria adopted for characterizing the experience
of subjects do not follow a standard and are frequently limited. Goal: Our
research aims at establishing an optimized and comprehensive scheme to
characterize the subjects' experience for studies in software engineering.
Method: Based on previous work, we defined the first version of this scheme,
composed of three experience attributes, including time, number of projects,
and self-perception. In the last years, we applied the characterization scheme
over four empirical studies, reaching the characterization of 79 subjects in
three different skills. Results: We found that the attributes from our scheme
are positively but moderately correlated. This finding suggests these
attributes play a complementary role in characterizing the subjects'
experience. Besides, we found that study subjects tend to enumerate the
technical diversity of their background when summarizing their professional
experience. Conclusion: The scheme proposed represents a feasible alternative
for characterizing subjects of empirical studies in the field. However, we
intend to conduct additional investigations with developers to evolve it.
