# 9 - Quantitative Methods in Systems Engineering
I'd like to discuss a little bit why we use methods and models. Throughout the lifecycle, methods and models are used for multiple purposes. Some of the types of decisions that we might make are defining the problem or opportunity space, deciding on concept architectures, and making trade off decisions. So what is trade off analysis? Well, it's a decision making activity, and it's for the purpose of selecting from alternative solutions on the basis of the benefit to the overall system and its stakeholders. So examples of the types of methods are decision matrix methods that let you compare alternatives in a fairly simple way, trade study method, which adds a bit more formality to this, and then trade space exploration methods, which allows you to explore the whole solution space. Trade studies for us is looking at how we can balance cost, performance, schedule, and even risk. It's gathering all that information and looking at those alternatives, and then coming up with ...