Hình Phân phối 1000 chênh lệch mẫu mô phỏng, mỗi tính bằng cách sử dụng ba quan sát rút ra một cách ngẫu nhiên 2 từ một bản phân phối bình thường với σ = 25. Trung bình của 1000 giá trị mô phỏng là 25,3, với 30 chênh lệch trên 100 và 190 phương sai của năm hoặc ít hơn. | TABLE Ninety Paired Measurements of Effluent Five-Day BOD and Effluent COD Concentrations COD BOD COD BOD COD BOD COD BOD COD BOD Note Concentrations are expressed as mg L. The regression is not strictly valid because both BOD and COD are subject to considerable measurement error. The regression correctly indicates the strength of a linear relation between BOD and COD but any statements about probabilities on confidence intervals and prediction would be wrong. Spearman Rank-Order Correlation Sometimes data can be expressed only as ranks. There is no numerical scale to express one s degree of disgust to odor. Taste appearance and satisfaction cannot be measured numerically. Still there are situations when we must interpret nonnumeric information available about odor taste appearance or satisfaction. The challenge is to relate these intangible and incommensurate factors to other factors that can be measured such as amount of chlorine added to drinking water for disinfection or the amount of a masking agent used for odor control or degree of waste treatment in a pulp mill. The Spearman rank correlation method is a nonparametric method that can be used when one or both of the variables to be correlated are expressed
