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CADDIS Volume 4: Data Analysis

Selecting an Analysis Approach

Establishing Differences from Expectations

Establishing Differences from Expectations

Establishing that biological or environmental characteristics at a test site differ from expected values is a key analysis for causal assessment. Expectations regarding site characteristics can be based on a single reference site (e.g., upstream of the test site) or on a set of comparable, regional reference sites. Analytical approaches range from a simple comparison of measurements to formal statistical tests.

  • Comparison of values

    If only a single measurement is available at the test site and at the reference site, then one can only compare these two values. Interpretation of whether the difference between the two values is meaningful requires an understanding of the inherent variability of the measurements and an understanding of ecologically meaningful differences in value.

  • Box plots

    Box plots graphically represent the distribution of a set of samples, providing a visual means of assessing whether a test site value deviates from the range of conditions observed at a single reference site or a set of similar reference sites.

  • Tests of significant difference

    Given enough data from different samples at a reference site, or data from several different reference sites, one can explicitly calculate the probability that the observation from the test site could have been observed at the reference site(s). A low probability (e.g. less than 5%) would suggest that the observation at the test site likely differs from expectations defined by reference sites.

  • Natural variations

    In cases where expectations are defined using a set of regional reference sites, it is likely that natural variations will be present in the characteristic that is being compared.

Interpreting results

All of these approaches provide some indication of whether test site characteristics differ from expectations.

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