The IPP Policy Brief 12, Assessing Budget Credibility: the case of public investment in Portugal (by João Matias), is the outcome of a partnership between Institute of Public Policy and International Budget Partnership, where the main goal was to learn more about why governments deviate from their budgets in particular areas and how they justify these deviations. The elaboration of this policy brief was intended to address credibility and budget transparency, and it was possible to show that much more work is needed to produce adequate explanations.
Globally, the project involved 23 civil society organizations and, in Portugal, the case analyzed was the underspending of the budgeted amounts of public investment. Some potential explanatory hypotheses were raised, such as the fact that public investment has been used as a adjustment variable.
You can also see more information in the Global Report (here) and in the case summaries (here).