Two-sided Test
In a two-sided (or two-tailed) test, the alternative hypothesis is non-directional: H₁: θ ≠ θ₀. The critical region is split between both tails, with α/2 in each tail. The critical values are more extreme than for one-sided tests, making two-sided tests less powerful but more conservative.
Real World
A psychologist investigating whether sleep deprivation affects reaction time uses a two-sided test because reaction time could plausibly speed up or slow down, so no direction is assumed in advance.
Exam Focus
For two-sided tests, halve α before looking up the critical value; forgetting this is one of the most common calculation errors.
How well did you know this?