Is this bad science? Since this is a retrospective study, there is non-random assignment for the treatment and control groups. That introduces selection bias. Projects that are more likely to implement a (long-winded) “waterfall” life-cycle approach are probably larger scale projects to begin with. Correlation is not causation. So, maybe it’s not the lifecycle approach that is the problem, but the confounding/lurking variable of project scale that is the problem. The study should control for the size of the project to make a valid conclusion about success rate of the development approach used. ie: Building a large insurance processing system will use a lifecycle approach, while building a fitness app will not. Apples to oranges, since one is much easier to be implement than the other.