Architecture-based development of software product lines requires well established methodologies for both development and assessment of software product line architectures. While several methodologies for the development of such systems have been proposed, their assessment has mostly relied on methods developed for single product architectures. In this paper, we extend the popular ATAM (Architecture Tradeoff Analysis Method) method to cover both the core architecture of the product line and the individual product architectures. In addition, the extended method prescribes a qualitative analytical treatment of variation points using scenarios. We present the main tenets of the new method and illustrate its use through a small case study.