The requirements for complex systems inevitably change continuously. Any successful software or information systems engineering approach needs to observe this simple fact. This paper argues for the critical importance of formative evaluation activities in any non-sequential, or learning-based RE process. We argue that evaluation with its focus on understanding and interpreting the evaluation results, is distinct from measurement. We also outline how evaluation activities can be performed from an interpretivist perspective, in a way that systematically informs formative evaluation activities during gradual, experimental requirements engineering activities.