Why are formulas used to describe substances

When is a hurricane considered to have made landfall? a. when at least one inch of rain occurs b. when the eye reaches the land mass c. when the outer wind bands make landfall d. when it moves inland from the coast

Aresearcher created three groups based on participants bmi: normal weight, overweight and obese. the hypothesis being tested is that the three groups differ in the mean number of artificially sweetened drinks consumed weekly. which statistical test might the researcher use, assuming a reasonably normal distribution of values.

Why are human population trends difficult to predict?