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CADDIS Volume 3

Quantile Regression: Non-insect Relative Abundance vs. Percent Sands and Fines for Minnesota Streams


  1. Analysis: Quantile regression
  2. Independent variable: Percentage (%), sand and fines
  3. Dependent variable: EPT (Ephemeroptera, Plecoptera and Trichoptera) abundance/m²
  4. Classification variables: Ecoregion class - dummy variable = 0 if the Ecoregions are 46, 47, or 51 and dummy variable = 1 if the Ecoregions are 50 or 52
  5. Key: symbols = observations, solid line = mean line, dashed line = 95% prediction limit, black lines and symbols = Ecoregions 46, 47 and 51, blue lines and symbols = Ecoregions 50 and 52
  6. Sample size: n = 267


  1. Model not plotted due to conflicting resultsa.
  2. Date: 6 October, 2006
Variable Estimate Chi2 p
Intercept 341.6      
Ecoregion class -93.0 2.74 0.098
% Sand & fines -1.88 6.08 0.014a
% Sand & fines x Ecoregion class 1.75 2.66  0.10

aHowever, Chi2 had a p > 0.05 (Chi2 = 1.09, p = 0.30) when "ecoregion class" was removed from the quantile regression.

Data Analysis

  1. Model generated using SAS Proc QUANTREG with the smoothing algorithm. The Wald test statistic was used to test the linear null hypothesis, H0: slope=0, for all independent variables (i.e., ecoregion class, % sand & fines, and ecoregion class x % sand & fines.)
  2. The ecoregions were aggregated into two classes because a preliminary ANOVA indicated that the base material for bedded sediments, % coarse substrates, was significantly greater in Ecoregions 50 and 52 than in Ecoregions 46, 47, and 51. Percent coarse substrates, however, did not significantly differ within either group.
  3. Replicate visits to a site were excluded from the data set.

Data Origin

  1. Agency: Minnesota Pollution Control Agency, St. Paul, Minnesota Exit
  2. Location: Minnesota - analyses restricted to wadeable sites (52 km² < watershed area < 523 km²) from the five ecoregions indicated below.
  3. Omernik Level III Ecoregions examined include: 46 (Northern Glaciated Plains), 47 (Western Corn Belt Plains), 50 (Northern Lakes and Forests), 51 (North Central Hardwoods), and 52 (Driftless Area).

Sampling Design

  1. Data collected 1996 through 2004.
  2. Sampling protocols: You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.


Michael B. Griffith,, (513) 569-7034

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