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CADDIS Volume 3

Linear Regression: Number of Salmonids vs. Metal Toxicity Units


  1. Analysis: Linear regression
  2. Independent variable: Metal Toxicity Units
  3. Dependent variable: Number of salmonids
  4. Classification variables: none
  5. Key: dot = observation, solid line = model, dotted line = 95% prediction interval, dashed line = 95% confidence interval
  6. Sample size: n = 30


  1. Number of salmonids = 64.367 - 48.763*(Metal Toxicity Units)
  2. r2 = 0.169
  3. Date: 12 November, 2006
Variable Estimate t value p
Intercept 64.367 3.922  
Metal Toxicity Units -48.763 -2.387 0.024

Data Analysis

  1. Model generated using SAS Proc GLM with the solution option. The grand mean and corrected sums of squares generated with Proc UNIVARIATE for calculating 95% prediction (eq 2.38a) and confidence intervals (eq 2.41) after Kutner et al. 2005.
  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.

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