  Background: A good search strategy is essential for a successful systematic
literature study. Historically, database searches have been the norm, which has
later been complemented with snowball searches. Our conjecture is that we can
perform even better searches if combining the two search approaches, referred
to as a hybrid search strategy. Objective: Our main objective was to compare
and evaluate a hybrid search strategy. Furthermore, we compared some
alternative hybrid search strategies to assess whether it was possible to
identify more cost-efficient ways of searching for relevant primary studies.
Method: To compare and evaluate the hybrid search strategy, we replicated an
SLR on industry-academia collaboration in software engineering. The SLR used a
more traditional approach to searching for relevant articles for an SLR, while
the replication was conducted using a hybrid search strategy. Results: In our
evaluation, the hybrid search strategy was superior in identifying relevant
primary studies. It identified 30 percent more primary studies and even more
when focusing only on peer-reviewed articles. To embrace individual viewpoints
when assessing research articles and minimise the risk of missing primary
studies, we introduced two new concepts, wild cards and borderline articles,
when conducting systematic literature studies. Conclusions: The hybrid search
strategy is a strong contender for being used when conducting systematic
literature studies. Furthermore, alternative hybrid search strategies may be
viable if selected wisely in relation to the start set for snowballing.
Finally, the two new concepts were judged as essential to cater for different
individual judgements and to minimise the risk of excluding primary studies
that ought to be included.
