Every day we are flooded with claims: “9 out of 10 dentists recommend this toothpaste,” “A new drug reduces heart attack risk by 30%,” “Students who sleep more get higher grades.” How do we know which claims to trust? The answer lies not in the numbers themselves, but in how the data behind them were gathered. This chapter is your guide to collecting data that can actually answer the question you care about — and to spotting when a study’s design makes its conclusions shaky.