Bias, Precision, Total ErrorBias, Precision, Total Error

Pat EllsworthPat Ellsworth

ITEPITEP

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Bias Error

• How far off, generally in one direction, your measurement is from the truth

• AKA: Systematic error

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Quantifying Bias Error

• Expressed as percentage (multiply by 100)If bias error = 0.02, express as 2%

• Bias error can be positive or negative

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Quantifying Bias Error (cont.)

• Find bias error• True ozone concentration is 80 ppb • Your measurement is 75 ppb

• What is the bias error?• Express as %

Bias Error

What might cause bias error?

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Precision Error

• How well your measurements of the same thing agree with each other

• AKA: Random error

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Quantifying Precision Error

two) the(of Average

BResult -A Result error PRECISION

• Expressed as percentageIf precision error = 0.02, express as 2%

• Precision error of 2 measurements in percent form can be called Relative Percent Difference, or RPD

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Quantifying Precision Error (cont.)

• Find precision error • First ozone measurement is 95 ppb • Second ozone measurement is 89 ppb

• What is the precision error?• What is the relative percent difference

(RPD)?

Precision Error

What might cause precision error?

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Bias Error & Precision Error: Examples

Small precision error

Large bias error

Moderate precision error

Large bias error

Small precision error

Small bias error

Large precision error

Small bias error (average of hits)

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Accuracy

• How much an observed value agrees with a reference or standard value

• Combination of precision error (random error) and bias error (systematic error)

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Total Error

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What does all of this have to do with me?

• EPA standards for monitoring

• Precision error <10%

• Bias error <10%

• Total Error ≤10% using measurement against a standard