Functions for estimating measurement uncertainty from standard uncertainties and either sensitivity coefficients or (for some methods) expressions or functions. Correlation is supported via either a correlation or covariance matrix.
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uncert(obj, ...) ## Default S3 method: uncert(obj, c, method = c("GUM", "MC"), cor, cov, distrib=NULL, distrib.pars=NULL, B=200, x=NULL, keep.x = TRUE, u=obj, ...) ## S3 method for class 'expression' uncert(obj, x, u, method=c("GUM", "NUM", "kragten", "k2", "MC"), cor, cov, distrib=NULL, distrib.pars=NULL, B=200, delta=0.01, keep.x = TRUE, ...) ## S3 method for class 'function' uncert(obj, x, u, method=c("NUM", "kragten", "k2", "MC"), cor, cov, distrib=NULL, distrib.pars=NULL, B=200, delta=0.01, keep.x = TRUE, ...) ## S3 method for class 'formula' uncert(obj, x, u, method=c("GUM", "NUM", "kragten", "k2", "MC"), cor, cov, distrib=NULL, distrib.pars=NULL, B=200, delta=0.01, keep.x = TRUE, ...)
An R object used for method dispatch; see below. Methods currently exist for numeric vector, expression, function, or formula
For the default method, a numeric vector of standard uncertainties. For
the formula or expression methods, a named list of standard uncertainties.Note that
for the default method,
A numeric vector of senstivity coefficients.
For the expression or formula methods, an R object which can be used
as an environment by
Method of uncertainty evaluation. The current list of methods is:
A (square, symmetric) correlation or covariance matrix, respectively.
If neither is specified,
Number of Monte Carlo replicates.
Step size for numerical differentiation.
Additional parameters to be passed to a function (for the function method) or used in an expression (for expression or formula method).
The default “GUM” method applies first-order error propagation principles to estimate
a combined standard uncertainty from a set of sensitivity coefficients and either a
set of standard uncertainties and a correlation matrix (which defaults to an identity matrix)
or a covariance matrix. Both options use the same calculation, which is simply
(t(c) %*% cov) %*% c ; standard uncertainties are first combined with
the correlation matrix provided to form the covariance matrix. Since the correlation
matrix defaults to the identity matrix, the default is combination without
The default method takes
obj as a vector of uncertainty contributions unless
is specified, in which case
u is used. It is not necessary to specify both.
The expression method requires
obj to be a differentiable R expression which can
be evaluated in the environment
x to provide a numeric value.
For the function method,
obj must be an R function which takes parameters from x and
returns a numeric value.
For the formula method,
obj must be a formula with no left-hand side (e.g.
which can be evaluated in the environment
x to provide a numeric value.
The formula and expression methods first calculate derivatives for the expression or formula,
evaluate them using the supplied values of
x and then pass the resulting sensitivity
coefficients, with supplied
cov to uncert.default.
The derivatives for the “GUM” method (formula and expression methods only) are algorithmic
derivatives (that is, algebraic or analytical derivatives) obtained using
Numerical derivatives are computed in different ways depending on the method specified:
method="NUM", the derivatives are calculated as
method="kragten", derivatives are calculated as
method="k2", derivatives are calculated as
"NUM" is likely to give a close approximation to analytical differentiation provided that
delta is appreciably less than 1 but not so small as to give step sizes near machine
"k2" is equivalent to
delta=1.0. Both will give zero coefficients
at stationary points (e.g minima), leading to under-estimation of uncertainty if
the curvature is large.
"kragten" uses a deliberately one-sided (and large) step to
avoid this problem; as a result,
"kragten" is a poorer (sometimes much poorer) estimate of
the analytical differential but likely a better approximation to the truth.
Since these methods rely on
u is unspecified and
u is extracted from
sqrt(diag(cov))). It is
assumed that the row and column order in
cov is identical to the order of named
Derivatives (and uncertainty contributions) are computed for all parameters in
x. Additional parameters used in
may be included in
...; these will be treated as constants in the
distrib is missing, or if it is a list with some members missing, the distribution
is assumed Normal and
distrib$name is set to
"norm". Similarly, if
or a member of it is missing, the default parameters for
list(mean=x$name, sd=u$name). If the list is not named,
names(x) are used
(so the list must be in order of
uncertMC. Distributions and
distribution parameters are required and B must be present and >1. See
for details of distribution specification.
For other evaluation methods, the distributions are silently ignored.
An object of class ‘uncert’ or, for
method="MC" of class ‘uncertMC’.
uncertMC-class for details.
S. L. R. Ellison email@example.com
JCGM 100 (2008) Evaluation of measurement data - Guide to the expression of uncertainty in measurement. http://www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf. (JCGM 100:2008 is a public domain copy of ISO/IEC Guide to the expression of uncertainty in measurement (1995) ).
Kragten, J. (1994) Calculating standard deviations and confidence intervals with a universally applicable spreadsheet technique, Analyst, 119, 2161-2166.
Ellison, S. L. R. (2005) Including correlation effects in an improved spreadsheet calculation of combined standard uncertainties, Accred. Qual. Assur. 10, 338-343.
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expr <- expression(a+b*2+c*3+d/2) x <- list(a=1, b=3, c=2, d=11) u <- lapply(x, function(x) x/10) u.expr<-uncert(expr, x, u, method="NUM") u.expr #Compare with default: uncert(u=c(0.1, 0.3, 0.2, 1.1), c=c(1.0, 2.0, 3.0, 0.5)) #... or with function method f <- function(a,b,c,d) a+b*2+c*3+d/2 u.fun<-uncert(f, x, u, method="NUM") u.fun #.. or with the formula method u.form<-uncert(~a+b*2+c*3+d/2, x, u, method="NUM") u.form #An example with correlation u.cor<-diag(1,4) u.cor[3,4]<-u.cor[4,3]<-0.5 u.formc<-uncert(~a+b*2+c*3+d/2, x, u, method="NUM", cor=u.cor) u.formc #A Monte Carlo example #See uncertMC for a less linear example u.formc.MC<-uncert(~a+b*2+c*3+d/2, x, u, method="MC", cor=u.cor, B=200) u.formc.MC
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