Engineering design and development of new products, which are either industrial products, technical innovations, hardware or software, often contain a very complex set of relationships among many coupled tasks. Controlling, redesigning and identifying features of these tasks can be usefully performed by a suitable model based on the design structure matrix in an iteration procedure. The proposed interval matrix model of design iteration controls and predicts slow and rapid convergence of iteration work on tasks within a project. A new model is based on Perron-Frobenius theorem and interval linear algebra where intervals and interval matrices are employed instead of real numbers and real matrices. In this way a more relaxed quantitative estimation of tasks is achieved and the presence of undetermined quantities is allowed to a certain extent. The presented model is demonstrated in the example of simplified software development process.