  Background: Accurate effort estimation is crucial for planning in Agile
iterative development. Agile estimation generally relies on consensus-based
methods like planning poker, which require less time and information than other
formal methods (e.g., COSMIC) but are prone to inaccuracies. Understanding the
common reasons for inaccurate estimations and how proposed approaches can
assist practitioners is essential. However, prior systematic literature reviews
(SLR) only focus on the estimation practices (e.g., [26, 127]) and the effort
estimation approaches (e.g., [6]). Aim: We aim to identify themes of reasons
for inaccurate estimations and classify approaches to improve effort
estimation. Method: We conducted an SLR and identified the key themes and a
taxonomy. Results: The reasons for inaccurate estimation are related to
information quality, team, estimation practice, project management, and
business influences. The effort estimation approaches were the most
investigated in the literature, while only a few aim to support the effort
estimation process. Yet, few automated approaches are at risk of data leakage
and indirect validation scenarios. Recommendations: Practitioners should
enhance the quality of information for effort estimation, potentially by
adopting an automated approach. Future research should aim to improve the
information quality, while avoiding data leakage and indirect validation
scenarios.
