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CADDIS Volume 3

Conditional Probability Analysis: Fish IBI < 40 vs. Total Suspended Solids for Iowa Streams


  1. Analysis: Conditional probability analysis
  2. Independent variable: Total suspended solids (TSS) (mg/L)
  3. Dependent variable: Probability of unacceptable condition for fish IBI at < 40 (out of 100)
  4. Key: symbols = observations
  5. Sample size: n = 420


  1. Conditional probability analysis does not produce a model equation. Rather, it plots the probabilities of observing the stated impairment (i.e. item 3 in the list above) over a stressor intensity gradient.
  2. Date: 8 November 2006

Data Analysis

  1. Program: S-Plus Version 7.0, user written scripts, equal weighting for site data
  2. One outlier removed (TSS > 220 and biology not consistent)

Data Origin

  1. Agency: Iowa Department of Natural Resources
  2. Program: TMDL for Little Floyd River
  3. Location: Little Floyd River, state of Iowa

Sampling Design

  1. Data collected using a targeted design from July 15 through October 15 over the years 1999 through 2001.
  2. Direct current electrofishing gear, standard operating procedures
  3. Methodology for Iowa's 2004 water quality assessment, listing, and reporting pursuant to Sections 305(b) and 303(d) of the federal Clean Water Act. Iowa Department of Natural Resources, Des Moines, Iowa (PDF) (88 pp, 309 K, About PDF)Exit


John F. Paul,, (919) 541-3160

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