Variance function

A variance function describes the relationship between the variance and the measured quantity value.

The are numerous models that describe the relationship between the variance and measured quantity value across the measuring interval:

Fit	Description
Constant variance	Fit constant variance across the measuring interval.
Constant CV	Fit constant coefficient of variation across the measuring interval.
Mixed constant / proportional variance	Fit constant variance at low levels with constant coefficient of variation at high levels.
2-parameter	Fir a 2-parameter linear variance function.
3-parameter	Fit Sadler 3-parameter variance function – a monotone relationship (either increasing or decreasing) between the variance and level of measurement.
3-parameter alternative	Fit Sadler alternate 3-parameter variance function – gives more flexibility than the Sadler standard 3-parameter variance function especially near zero.
4-parameter	Fit a Sadler 4-parameter variance function - allows for a turning point near the detection limit.

A variance function can be useful to estimate the limit of detection or limit of quantitation.

Related concepts

Precision profile plot

Related tasks

Fitting a variance function

Related information

Sadler, W. A., & Smith, M. H. (1986). A reliable method of estimating the variance function in immunoassay. Computational Statistics & Data Analysis, 3, 227-239.

Sadler, W. A., Smith, M. H., & Legge, H. M. (1988). A method for direct estimation of imprecision profiles, with reference to immunoassay data. Clinical chemistry, 34(6), 1058-1061.

Sadler, W. A., (2016). Using the variance function to estimate limit of blank, limit of detection and their confidence intervals. Ann Clin Biochem, 53, 141-149.

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