Sample size is not the only factor that can make it more difficult to detect an existing difference. Suppose you are assessing length differences on average between male and female crabs, using a millimeter scale.

• Scenario 1: The major claw of male and female fiddler crabs differs in length by an average of 4 mm. Large effect.
• Scenario 2: The major claw of male and female fiddler crabs differs in length by an average of 0.04 mm. Small effect.

The difference in Scenario 1 is much easier to detect than the difference in Scenario 2. Typically, effects are determined to be small, medium, or large. Large effects are easier to detect than medium or small effects. Detecting a small effect will often require a larger sample size than detecting a large effect.