One of the things we should always watch for, is the Business Case. It’s one of the seven themes in PRINCE2.
Business Case
The Business Case is a concept that contains the justification of the project, and its supportive information. We use it for making high-level decisions, and to see if it’s still beneficial to continue the project.

Do you remember the Manage by Stages principle? OK, let’s discuss the Business Case based on the stages:

    Pre-project: it’s not a stage, but a period before we start the project. We just want to know if it’s a good idea to start the project. So, we create a simple Business Case. A complete and reliable Business Case needs a lot of planning; for example, we should know the resource requirements and schedule. But it’s really expensive to do so for every idea. That’s why we create an “outline Business Case” in the pre-project period, which is really rough and high-level. If it was promising, we’ll move on to the first “stage”.
    Initiation stage: the first stage is initiation, and it simply means “planning”. In this stage, we prepare a high-level plan for the whole project. Therefore, we would be able to turn our “outline Business Case” into a real “Business Case”. Then we check to see if the project is beneficial enough, and if it was, we will start the second stage, which is the first delivery stage.
    Delivery stages: we will have one or more delivery stages after the initiation stage. During them, we will keep updating the business case and use it to make the high-level decisions and see if we need to cancel the project.

It’s easy to talk about updating the Business Case, but it needs a lot of work. So, when should we do it? Every week for example? Of course not. What’s your idea? Tip: we do it for two events.