Statistics for Environmental Engineers

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Draper, N. R. and H. Smith (1998). Applied Regression Analysis, 3rd ed., New York, John Wiley.

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39.1 COD Calibration. The ten pairs of readings below were obtained to calibrate a UV spectrophotometer to measure chemical oxygen demand (COD) in wastewater.

COD (mg/L)    60    90    100    130    195    250    300    375    500    600

UV Absorbance    0.30    0.35    0.45    0.48    0.95    1.30    1.60    1.80    2.3    2.55

2 2

(a) Fit a linear model to the data and obtain the R value. (b) Discuss the meaning of R in the context of this calibration problem. (c) Exercise 36.3 contains a larger calibration data set for the same instrument. (d) Fit the model to the larger sample and compare the values of R2. Will the calibration curve with the highest R2 best predict the COD concentration? Explain why or why not.

39.2 Stream pH. The data below are n = 200 monthly pH readings on a stream that cover a period of almost 20 years. The data read from left to right. The fitted regression model is y = 7.1435 —

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