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CADDIS Volume 3

Conditional Probability Analysis: Chlorophyll a > 30 vs. Total Phosphorus for Northeast Lakes


  1. Analysis: Conditional probability analysis
  2. Independent variable: Total phosphorus concentration in upper water column (ug/L)
  3. Dependent variable: Probability of unacceptable condition for chlorophyll a at > 30 (ug/L)
  4. Key: symbols = observations, dashed line = 95% Confidence Intervals
  5. Sample size: n = 483


  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, variable weighting for site data

Data Origin

  1. Agency: U.S. Environmental Protection Agency
  2. Program: Environmental Monitoring and Assessment Program (EMAP).
  3. Project: Surface Waters, Northeast Lakes Data
  4. Location: Northeast United States

Sampling Design

  1. Data collected using the EMAP probability design over July through September in 1991 through 1994.
  2. Single grab sample at 1.5 m below surface with van Dorn sampler according to Baker, JR, GD Merritt and DW Sutton (eds.). 1997. Environmental Monitoring and Assessment Program - Surface Waters: Field Operations Manual for Lakes.
  3. Catalog Documentation for EMAP Surface Waters Northeast Lakes Data, EMAP.


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

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