Statistics for Environmental Engineers

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(b)    Estimate the median and the trimmed mean.


(c)    Estimate the population mean and standard deviation by computing the Winsorized mean and standard deviation.


15.2 Lead in Tap Water. The data below are lead measurements on tap water in an apartment complex. Of the total n = 140 apartments sampled, 93 had a lead concentration below the limit of detection of 5 Ug/L. Estimate the median lead concentration in the 140 apartments. Estimate the mean lead concentration.


Pb (Ug/L) 0-4.9 5.0-9.9 10-14.9 15-19.9 20-29.9 30-39.9 40-49.9 50-59.9 60-69.9 70-79.9 Number 93    26    6    4    7    1    1    1    0    1


15.3 Lead in Drinking Water. The data below are measurements of lead in tap water that were sampled early in the morning after the tap was allowed to run for one minute. The analytical limit of detection was 5 Ug/L, but the laboratory has reported values that are lower than this. Do the values below 5 Ug/L fit the pattern of the other data? Estimate the median and the 90th percentile concentrations.


Pb fog/L)


Number


%


Cum. %


0-0.9


20


0.143


0.143


1-1.9


16


0.114


0.257


2-2.9


32


0.229


0.486


3-3.9


11


0.079


0.564


4-4.9


13


0.093


0.657


5-9.9


27


0.193


0.850


10-14.9


7


0.050


0.900


15-19.9


4


0.029


0.929


20-29.9


6


0.043


0.971


30-39.9


1


0.007


0.979

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