If the standard deviation is small, we define a new variable known as

When the population standard deviation is unknown and the sample size is small, the t-distribution (Student's t-distribution) is used. The t-variable is defined as (x̄ - μ)/(s/√n), where s is the sample standard deviation. The phrase 'standard deviation is small' likely refers to the scenario where the sample standard deviation is used as an estimate, leading to the t-distribution.