epiR (version 2.0.68)

epi.betabuster: An R version of Wes Johnson and Chun-Lung Su's Betabuster

Description

A function to return shape1 and shape2 parameters for a beta distribution, based on expert elicitation.

Usage

epi.betabuster(mode, conf, greaterthan, x, conf.level = 0.95, max.shape1 = 100, 
   step = 0.001)

Value

A list containing the following:

shape1

the shape1 parameter for the estimated beta distribution.

shape2

the shape2 parameter for the estimated beta distribution.

mode

the mode of the estimated beta distribution.

mean

the mean of the estimated beta distribution.

median

the median of the estimated beta distribution.

lower

the lower bound of the confidence interval of the estimated beta distribution.

upper

the upper bound of the confidence interval of the estimated beta distribution.

variance

the variance of the estimated beta distribution.

Arguments

mode

scalar, the mode of the variable of interest. Must be a number between 0 and 1.

conf

level of confidence (expressed on a 0 to 1 scale) that the true value of the variable of interest is greater or less than argument x.

greaterthan

logical, if TRUE you are making the statement that you are conf confident that the true value of the variable of interest is greater than x. If FALSE you are making the statement that you are conf confident that the true value of the variable of interest is less than x.

x

scalar, value of the variable of interest (see above).

conf.level

magnitude of the returned confidence interval for the estimated beta distribution. Must be a single number between 0 and 1.

max.shape1

scalar, maximum value of the shape1 parameter for the beta distribution.

step

scalar, step value for the shape1 parameter. See details.

Author

Simon Firestone (Melbourne Veterinary School, Faculty of Science, The University of Melbourne, Parkville Victoria 3010, Australia) with acknowledgements to Wes Johnson and Chun-Lung Su for the original standalone software.

Details

The beta distribution has two parameters: shape1 and shape2, corresponding to a and b in the original version of BetaBuster. If r equals the number of times an event has occurred after n trials, shape1 = (r + 1) and shape2 = (n - r + 1).

Take care when you're parameterising probability estimates that are at the extremes of the 0 to 1 bounds. If the returned shape1 parameter is equal to the value of max.shape1 (which, by default is 100) consider increasing the value of the max.shape1 argument. The epi.betabuster functions issues a warning if these conditions are met.

References

Christensen R, Johnson W, Branscum A, Hanson TE (2010). Bayesian Ideas and Data Analysis: An Introduction for Scientists and Statisticians. Chapman and Hall, Boca Raton.

Su C-L, Johnson W (2014) Beta Buster. Software for obtaining parameters for the Beta distribution based on expert elicitation. URL: https://cadms.vetmed.ucdavis.edu/diagnostic/software.

Examples

## EXAMPLE 1:
## If a scientist is asked for their best guess for the diagnostic sensitivity
## of a particular test and the answer is 0.90, and if they are also willing 
## to assert that they are 80% certain that the sensitivity is greater than 
## 0.75, what are the shape1 and shape2 parameters for a beta distribution
## satisfying these constraints? 

rval.beta01 <- epi.betabuster(mode = 0.90, conf = 0.80, greaterthan = TRUE, 
   x = 0.75, conf.level = 0.95, max.shape1 = 100, step = 0.001)
rval.beta01$shape1; rval.beta01$shape2

## The shape1 and shape2 parameters for the beta distribution that satisfy the
## constraints listed above are 9.875 and 1.986, respectively.

## This beta distribution reflects the probability distribution obtained if 
## there were 9 successes, r:
r <- rval.beta01$shape1 - 1; r

## from 10 trials, n:
n <- rval.beta01$shape2 + rval.beta01$shape1 - 2; n

dat.df01 <- data.frame(x = seq(from = 0, to = 1, by = 0.001), 
   y = dbeta(x = seq(from = 0, to = 1,by = 0.001), 
   shape1 = rval.beta01$shape1, shape2 = rval.beta01$shape2))

## Density plot of the estimated beta distribution:

if (FALSE) {
library(ggplot2)

ggplot(data = dat.df01, aes(x = x, y = y)) +
   geom_line() + 
   scale_x_continuous(name = "Test sensitivity") +
   scale_y_continuous(name = "Density")
}


## EXAMPLE 2:
## The most likely value of the specificity of a PCR for coxiellosis in 
## small ruminants is 1.00 and we're 97.5% certain that this estimate is 
## greater than 0.99. What are the shape1 and shape2 parameters for a beta 
## distribution satisfying these constraints?

epi.betabuster(mode = 1.00, conf = 0.975, greaterthan = TRUE, x = 0.99, 
   conf.level = 0.95, max.shape1 = 100, step = 0.001)

## The shape1 and shape2 parameters for the beta distribution that satisfy the
## constraints listed above are 100 and 1, respectively. epi.betabuster 
## issues a warning that the value of shape1 equals max.shape1. We increase
## max.shape1 to 500:

epi.betabuster(mode = 1.00, conf = 0.975, greaterthan = TRUE, x = 0.99, 
   conf.level = 0.95, max.shape1 = 500, step = 0.001)

## The shape1 and shape2 parameters for the beta distribution that satisfy the
## constraints listed above are 367.04 and 1, respectively.