Write hypotheses and select a tail

A hypothesis is a statement about a population parameter or model. The null H0 provides the reference value or no-effect claim. The alternative H1 states the effect or difference to be investigated. A directional alternative uses > or <; a nondirectional alternative uses ≠. Choose direction before seeing the sample result. For a two-sided 5% z test, the critical values are ±1.96, whereas a one-sided 5% test uses 1.645 in the specified tail.