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Quantile Regression: EPT Taxa Richness vs. Metals Toxicity Units for Colorado Streams

 Toxicity Units - Metals vs EPT Taxa Richness


  1. Analysis: Quantile regression
  2. Independent variable: Metal Toxicity Units
  3. Dependent variable: EPT Taxa Richness
  4. Key: dot=observation; line= 90th quantile
  5. Sample size: n = 37


  1. EPT Taxa Richness = 22.565 - 8.708 (Metals-Toxicity Units)
  2. Date: 12 November, 2006
VariableEstimatelower CLupper CLTest TypeStatisticp
Intercept 22.565 17.087 28.043 T value 8.362  
Metal Toxicity Units -8.708 -16.112 -1.303 T value -2.387 0.022
Wald       Chi2 5.700 0.017
Likelihood Ratio       Chi2 9.787 0.002

Data Analysis

  1. Model generated using SAS Proc QUANTREG with the smoothing algorithm. The WALD and Likelihood test statistics were used to test the linear null hypothesis, H0: slope=0.
  2. Analysis for sites classified as pH neutral, but with strong AMD input.
  3. Metal Toxicity Units (TU) were calculated as sum of the ratios of each measured metal concentration with the criterion for that metal.
  4. Replicate visits to a site were not excluded from the data set.

Data Origin

  1. Agency: U.S. Environmental Protection Agency
  2. Program: Environmental Monitoring and Assessment Program (EMAP)
  3. Project: Colorado Metals Belt Stream Assessment (EPA Region 8)
  4. Filenames: Files benmet, fishmet and chem downloaded from the Surface Waters Information Management (S.W.I.M.S.) database web site.

Sampling Design

  1. Data collected 1994 through 1995.
  2. Analyzed metals and metalloids: aluminum, arsenic, cadmium, chromium, copper, iron, manganese, nickel, lead, selenium, silver, zinc.
  3. A description of the sampling methods may be found in the EMAP West Methods Manual for Streams found on the S.W.I.M.S. database web site.


Patricia Shaw-Allen,, (513) 569-7136

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