How do you decide whether a new medicine really works, or if an advertising campaign lifted sales, or if a coin is fair? You need a clear, repeatable way to weigh evidence and make a decision. This chapter introduces the core ideas that let you turn data into a yes‑or‑no answer while controlling how often you are wrong. We start with simple logic and then move to the most common tests for normal data. By the end, you can design and interpret basic statistical tests yourself.