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

### Plot

- Analysis: Quantile regression
- Independent variable: Percentage (%), sand and fines
- Dependent variable: Non-insect abundance/m²
- 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
- 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
- Sample size: n = 267

### Model

- Ecoregions 50 and 52: Non-insect abundance = 67.00 + 1.17(% sand & fines)
- Ecoregions 46, 47 and 51: Non-insect abundance = 82.58 + 3.28(% sand & fines)
- Date: 6 October, 2006

Variable | Estimate | Chi^{2} | p |
---|---|---|---|

Intercept | 82.58 | ||

Ecoregion class | -15.58 | 0.08 | 0.77 |

% Sand & fines | 3.284 | 14.49 | 0.0001 |

% Sand & fines x Ecoregion class | -2.114 | 5.21 | 0.022 |

### Data Analysis

- Model generated using SAS Proc QUANTREG with the smoothing algorithm. The Wald test statistic was used to test the linear null hypothesis, H
_{0}: slope=0, for all independent variables (i.e., ecoregion class, % sand & fines, and ecoregion class x % sand & fines.) - 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.
- Replicate visits to a site were excluded from the data set.

### Data Origin

- Agency: Minnesota Pollution Control Agency , St. Paul, Minnesota
- Location: Minnesota - analyses restricted to wadeable sites (52 km
^{²}< watershed area < 523 km^{²}) from the five ecoregions indicated below. - 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

- Data collected 1996 through 2004.
- Sampling protocols:
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### Contact

Michael B. Griffith, griffith.michael@epa.gov, (513) 569-7034