Simplify the following text for a Grade 9 native English-speaking person.
Original: Most of decision theory is normative or prescriptive , i.e. , it is concerned with identifying the best decision to take , assuming an ideal decision maker who is fully informed , able to compute with perfect accuracy , and fully rational .
Simplified: In most cases , decision theory is normative and prescriptive . Most of the time , it assumes the decision maker has perfect knowledge .