When testing μ, how do we decide whether to use the standard normal distribution or a Student's t distribution? If the x distribution is normal and n ≥ 30 with known σ we use the Student's t. With the same conditions and unknown σ we use the standard normal. If the x distribution is normal and n ≥ 20 with known σ we use the standard normal. With the same conditions and unknown σ we use the Student's t. If the x distribution is normal and n ≥ 30 with known σ we use the standard normal. With the same conditions and unknown σ we use the Student's t. Whenever we assume the populations are independent, we always use the standard normal.