  Analysing and improving productivity has been one of the main goals of
software engineering research since its beginnings. A plethora of studies has
been conducted on various factors that resulted in several models for analysis
and prediction of productivity. However, productivity is still an issue in
current software development and not all factors and their relationships are
known. This paper reviews the large body of available literature in order to
distill a list of the main factors influencing productivity investigated so
far. The measure for importance here is the number of articles a factor is
mentioned in. Special consideration is given to soft or human-related factors
in software engineering that are often not analysed with equal detail as more
technical factors. The resulting list can be used to guide further analysis and
as basis for building productivity models.
