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Format a Confidence Interval

Source: R/misc.R
formatCI.Rd

This formats a confidence interval in either the +/- notation or the parenthetic notation. For example, if a quantity has mean 1 with uncertainty 0.05, which means a CI of 0.95 to 1.05, the "+-" style yields "1+/-0.05", and the "parentheses" style yields `""'.

Usage

formatCI(
  ci,
  style = c("+/-", "parentheses"),
  model,
  digits = 2,
  debug = getOption("oceDebug", 0)
)

Arguments

ci

optional vector of length 2 or 3.

style

string indicating notation to be used.

model

optional regression model, e.g. returned by lm() or nls().

digits

optional number of digits to use. This is ignored if style is "parentheses".

debug

integer value indicating debugging level. If 0, then formatCI() works silently. If greater than 0, then some debugging messages are printed during processing.

Value

If ci is given, the result is a character string with the estimate and its uncertainty, in plus/minus or parenthetic notation. If model is given, the result is a 1-column matrix holding character strings, with row names corresponding to the parameters of the model.

Details

If a model is given, then ci is ignored, and a confidence interval is calculated using confint() with level set to 0.6914619. This level corresponds to a range of plus or minus one standard deviation, for the t distribution and a large number of degrees of freedom (since qt(0.6914619, 100000) is 0.5).

If model is missing, ci must be provided. If it contains 3 elements, then first and third elements are taken as the range of the confidence interval (which by convention should use the level stated in the previous paragraph), and the second element is taken as the central value. Alternatively, if ci has 2 elements, they are taken to be bounds of the confidence interval and their mean is taken to be the central value.

In the +/- notation, e.g. \(a \pm b\) indicates that the true value lies between \(a-b\) and \(a+b\) with a high degree of certainty. Mills et al. (1993, section 4.1 on page 83) suggest that \(b\) should be set equal to 2 times the standard uncertainty or standard deviation. JCGM (2008, section 7.2.2 on pages 25 and 26), however, suggest that \(b\) should be set to the standard uncertainty, while also recommending that the \(\pm\) notation (and presumably the parentheses notation also) be avoided altogether, in favour of writing sentences that explains uncertainties in clear terms.

The parentheses notation is often called the compact notation. In it, the digits in parentheses indicate the uncertainty in the corresponding digits to their left, e.g. 12.34(3) means that the last digit (4) has an uncertainty of 3. However, as with the \(\pm\) notation, different authorities offer different advice on defining this uncertainty; Mills et al. (1993) provide an example in which the parenthetic value is half the \(\pm\) value, whereas JCM (2008) suggest using the same values.

The JCM(2008) convention is used by formatCI() for the parentheses notation, as illustrated in Examples 1 and 2. Note, however, that if the confidence range exceeds the value, then a request for parentheses format reverts to +/- format.

References

  1. JCGM, 2008. Evaluation of measurement data - Guide to the expression of uncertainty in measurement (JCGM 100:2008), published by the Joint Committee for Guides in Metrology, available (as of November 2023) at https://www.bipm.org/documents/20126/2071204/JCGM_100_2008_E.pdf. See section 7.2.2 on Page 25, for a summary of notation, including an illustration of the use of equal values for both the +- and the parentheses notations.

  2. Mills, I., T. Cvitas, K. Homann, N. Kallay, and K. Kuchitsu, 1993. Quantities, Units and Symbols in Physical Chemistry, published Blackwell Science for the International Union of Pure and Applied Chemistry. (See section 4.1, page 83, for a summary of notation, which shows that a value to the right of a +- sign is to be halved if put in in parentheses, which is not done in the present function, because of a choice to follow the recommendation of reference 1.

Author

Dan Kelley

Examples

library(oce)

# Example 1: mean=1, uncertainty=0.05, in +/- notation.
formatCI(c(0.95, 1.05)) # "1+/-0.05"
#> [1] "1+/-0.05"

# Example 2: save mean and uncertainty, but in parentheses notation.
formatCI(c(0.95, 1.05), style = "parentheses") # "1.00(5)"
#> [1] "1.00(5)"

# example 3: using t.test to find a CI.
a <- rnorm(100, mean = 10, sd = 1)
CI <- t.test(a)$conf.int
formatCI(CI)
#> [1] "9.9+/-0.19"
formatCI(CI, style = "parentheses")
#> [1] "9.9(1.87)"

# example 4: specifying a model
x <- seq(0, 10, 0.1)
y <- 2 + 3 * x + rnorm(x, sd = 0.1)
m <- lm(y ~ x)
formatCI(model = m)
#>             value       
#> (Intercept) "2+/-0.021" 
#> x           "3+/-0.0036"
formatCI(model = m, style = "parentheses")
#>             value        
#> (Intercept) "1.97(2.11)" 
#> x           "3.005(3.64)"