from at least 15 consecutive Test Strengths for concrete made with the same mix proportions
from at least 30 Test Strengths, comprising two sets of consecutive Test Strengths.
ACI 301-96 (References 1) (clause 4.2.3.2.a) stipulates the values used to calculate the Standard Deviation of the Test Strengths should be obtained
from at least 15 consecutive Test Strengths for concrete made with the same mix proportions
from at least 30 Test Strengths, comprising two sets of consecutive Test Strengths.