Stating that there is not an effect or difference when one exists (null hypothesis is not rejected when it is in fact false).Also known as false-negative error.

β is the probability of making a type II error. β is related to statistical power (1-β), which is the probability of rejecting the null hypothesis when it is false.

↑ power and ↓ β by:- ↑ sample size- ↑ expected effect size- ↑ precision of measurement