# dod: Thurstonian Degree-of-Difference (DOD) model In perbrock/sensR: Thurstonian Models for Sensory Discrimination

 dod R Documentation

## Thurstonian Degree-of-Difference (DOD) model

### Description

Fits the Thurstonian Degree-of-Difference (DOD) model and performs hypothesis/significance tests of d-prime (Thurstonian delta). One-sided difference and similarity tests as well as two-sided tests of d-prime are available. The user may choose from a number of tests statistics.

### Usage

dod(same, diff, d.prime0 = 0, conf.level = 0.95,
statistic = c("likelihood", "Pearson", "Wilcoxon", "Wald"),
alternative = c("difference", "similarity", "two.sided",
"less", "greater"), control=dodControl(), ...)

## S3 method for class 'dod'
print(x, digits = max(3, getOption("digits") - 3), ...)

### Arguments

 same the answers to same-pairs; either 1) a numeric vector of counts of length equal to the number of response categories ordered appropriately or 2) a factor where the levels indicate the response categories. diff the answers to different-pairs in the same format as same. d.prime0 the value of d.prime under the null hypothesis. In the standard no-difference test d.prime0 = 0, while it has to be positive for similarity tests and two-sided tests. conf.level the confidence level for the confidence intervals statistic the statistic to be used for hypothesis testing alternative the nature of the alternative hypothesis in the hypothesis/significance test for d-prime. Note that "greater" is an alias for "difference" and "less" is an alias for "similarity" control options to control the fitting process specfied via a call to dodControl. x an object of class "dod". digits number of digits in resulting table of results. ... not currently used.

### Details

dod will report the likelihood based confidence interval for d.prime unless statistic = "Wald" in which case the standard symmetric Wald type confidence interval is reported. This interval can be highly inaccurate and so is not recommened for practical use.

The p-value for the standard one-tailed difference test of "no difference" is obtained with d.prime0 = 0 corresponding to the default setting.

The standard error of d-prime is not defined when the parameter estimate is zero (or numerically close) and it will be reported as NA in this case.

The "Wald" statistic is *NOT* recommended for practical use—it is only included here for completeness and to allow comparisons with other software etc.

### Value

An object of class dod.

### Author(s)

Rune Haubo B Christensen

### References

Ennis, J.M. and R.H.B. Christensen (2015) A Thurstonian comparison of the tetrad and degree of difference tests. Food Quality and Preference, 40, pp.263-269.

Christensen, R.H.B, J.M. Ennis, D.M. Ennis and P.B Brockhoff (2012) A Thurstonian model for the Degree of Difference test with extensions to unequal variance, sequence effects and replicated data. Talk at Sensometrics conference, Rennes, France, July 11th.

dodSim, dodPwr, dodControl, dod_fit, optimal_tau

### Examples

## DOD example data:
same.pairs <- c(25, 22, 33, 20)
diff.pairs <- c(18, 22, 30, 30)

## Fit Thurstonian dod-model and perform difference test:
dod(same=same.pairs, diff=diff.pairs)

## Can choose another test statistic (e.g. Wilcoxon):
dod(same=same.pairs, diff=diff.pairs, statistic="Wilcox")

## A similarity test (with simulated data):
set.seed(121)
(Data2 <- dodSim(d.prime=0, ncat=4, sample.size=200, method.tau="equi.prob"))
dod(same=Data2[1, ], diff=Data2[2, ], d.prime0=1.2,
alternative="similarity")

## Extract parameters from a dod fit:
fm <- dod(same=same.pairs, diff=diff.pairs)
coef(fm)

perbrock/sensR documentation built on Nov. 5, 2023, 10:41 a.m.