美國凝凝土學會ACI 214混凝土規範假設凝凝土抗壓強度試驗數據會呈現常態分佈曲線，再根據常態分佈理論，使用標準偏差導出混凝土配比目標需求強度計算公式。另一方面，ACI 214也使用變異係數導出混凝土配比目標需求強度之替代計算公式。然而，實際產製的混凝土其抗壓強度試驗資料未必呈現左右對稱的常態分佈曲線圖。本文使用常態分佈理論之標準偏差法、雙異係數法及對數常態分佈理論三種方法，以混凝土廠實際案例討論混凝土目標需求強度公式之應用，本文也建議以修正發生頻率常數之方式，得到較合適的混凝土配比目標需求強度公式，以得到符合設計安全要求且產製成本較低之混凝土。

ACI 214 provides a statistical analysis for cylinder strength evaluation. ACI 214 derives the required average strength equations for concrete by using standard deviations. ACI 214 also provides the alternative required average strength equations for concrete by using coefficient of variation. Both approaches are based on normal distribution. But, the experimental data are not exact normal distribution. In present paper, the approach based on lognormal distribution are discussed and coMPared with the approaches based on standard deviations and the coefficient of variation of normal distribution. The experimental data are in situ data. Also, the modified required average strength equations by using a modified probability constant are proposed. The modified required average strength equations will lead to safer and cheaper concrete products.