PMP Practice: Balance competing resource demands

Katarina Petrović-Jovanović is managing a hybrid project for Sabai Sabai Digital to implement a new client portal. The project uses two-week iterations for software development and a predictive approach for infrastructure deployment. During the third iteration, the development team's velocity drops significantly, and the product owner adds several unplanned features that were not in the iteration backlog. At the end-of-iteration review, Lila discovers the team has consumed 85% of the iteration budget but completed only 40% of planned story points. The infrastructure deployment is still on track with its original budget. Lila needs to understand what caused the budget variance before the next iteration begins in three days. Which approach would be MOST effective for Lila to address this situation?