D-error is a measure that quantifies how good or bad a design is at extracting information from respondents in an experiment. A lower D-error indicates a better design. A design that minimizes D-error is considered D-optimal. Many other related measures exist that also serve this purpose, such as D-optimality. This article describes computing D-error, Bayesian D-error, and D-efficiency.
Computing D-error
I'll show you how to compute D-error to assess the quality of your experiment. There are many different types of experiments. One type is a choice experiment, which represents a design with a table where each row corresponds to a choice alternative and the columns correspond to the version, task, question, alternative, and attributes. I've shown you an example of a design below:
| Version | Task | Question | Alternative | Attribute 1 | Attribute 2 | Attribute 3 |
|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 | 2 | 1 |
| 1 | 1 | 1 | 2 | 2 | 1 | 2 |
| 1 | 2 | 2 | 1 | 1 | 2 | 2 |
| 1 | 2 | 2 | 2 | 2 | 1 | 1 |
| 1 | 3 | 3 | 1 | 2 | 2 | 1 |
| 1 | 3 | 3 | 2 | 1 | 1 | 2 |
| 2 | 4 | 1 | 1 | 2 | 2 | 2 |
| 2 | 4 | 1 | 2 | 1 | 1 | 1 |
| 2 | 5 | 2 | 1 | 2 | 2 | 2 |
| 2 | 5 | 2 | 2 | 1 | 1 | 1 |
| 2 | 6 | 3 | 1 | 1 | 2 | 1 |
| 2 | 6 | 3 | 2 | 2 | 1 | 2 |
To compute D-error, first encode the levels of the attributes in the design using dummy coding (or alternatively effects-type coding) and split the encoded design by questions (suppose there are Q of them) to create a set of matrices
![\[ {\textbf{X}}={\mathbit{\textbf{X}}_1,\cdots,\mathbit{\textbf{X}}_Q}. \]](data:image/svg+xml;nitro-empty-id=NjIyOjgxMQ==-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTM1IDE4IiB3aWR0aD0iMTM1IiBoZWlnaHQ9IjE4IiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
In addition, you need to make a prior assumption regarding the respondent parameters. The different possible assumptions lead to three variants of the D-error. The first is the Dp-error, which assumes that all respondent parameters are given by a parameter vector β . The multinomial logit probability that a respondent chooses alternative 𝑖 in question 𝑞 is
![\[ \textbf{p}_{q,i}=\frac{\exp\funcapply(\textbf{X}_{q,i}\mathbit{\beta})}{\sum_{j=1}^{J}{\exp\funcapply(\textbf{X}_{q,j}\mathbit{\beta})}} \]](data:image/svg+xml;nitro-empty-id=NjI0OjEyMTE=-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTg1IDQ5IiB3aWR0aD0iMTg1IiBoZWlnaHQ9IjQ5IiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
where J is the number of alternatives per question and 
is a vector of length J.
We first construct the Fisher information matrix M using the formula below
![\[ {\textbf{M}}({\textbf{X}},{\beta})=\sum_{q=1}^{Q}{{\textbf{X}}^\prime}_q({\textbf{P}}_q-{\textbf{p}}_q{\textbf{p}}_q\prime)}{\textbf{X}}_q \]](data:image/svg+xml;nitro-empty-id=NjI3OjEyMjg=-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMjY5IDU2IiB3aWR0aD0iMjY5IiBoZWlnaHQ9IjU2IiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
where
![\[ {\textbf{P}}_q=\textrm{diag}({\textbf{p}}_\mathbit{q}).\]](data:image/svg+xml;nitro-empty-id=NjI5Ojc2MA==-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTE0IDIxIiB3aWR0aD0iMTE0IiBoZWlnaHQ9IjIxIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
Dp-error is a function of the determinant of the information matrix
![\[ D_P(\textbf{X})=\left|\textbf{M}(\textbf{X},{\beta})\right|^{-1/K} \]](data:image/svg+xml;nitro-empty-id=NjMxOjgxMQ==-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTk0IDIzIiB3aWR0aD0iMTk0IiBoZWlnaHQ9IjIzIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
where K is the number of parameters in β.
D0-error
The simpler D0-error assumes that respondents have no preference for any of the attribute levels, which is equivalent to assuming that β=0. The logit probabilities 
become 1/J for all questions and alternatives. The information matrix formula in this case reduces to
![\[ \textbf{M}(\textbf{X})=\sum_{q=1}^{Q}{{\textbf{X}^\prime}_q\textbf{X}_q-J^{-1}({\textbf{X}^\prime}_q\textbf{1}_J)({\textbf{1}^\prime}_J\textbf{X}_q)} \]](data:image/svg+xml;nitro-empty-id=NjM1OjEyODE=-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMzIzIDU2IiB3aWR0aD0iMzIzIiBoZWlnaHQ9IjU2IiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
where 1J is a vector of J ones. Apart from this difference, the D0-error is computed using the same function of the determinant as for DP-error. A design created by optimizing D0-error is known as a utility-neutral design.
DB-error
Lastly, DB-error assumes that respondent parameters are distributed according to a probability distribution, which is often a multivariate normal distribution with a diagonal covariance matrix.
DB-error is simply the integral of DP-error over the prior distribution of respondent parameters
![\[ D_B(\textbf{X})=\int{\log\funcapply\left|\textbf{M}\left(\textbf{X},\mathbit{\beta}\right)\right|\pi(\mathbit{\beta})d\mathbit{\beta}} \]](data:image/svg+xml;nitro-empty-id=NjQwOjExOTk=-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMjY1IDQwIiB3aWR0aD0iMjY1IiBoZWlnaHQ9IjQwIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
where π(β) denotes the density of the prior distribution at β. A design created by optimizing DB-error is also known as a Bayesian design.
D-efficiency
D-efficiency is a relative measure based on comparing one design X against a benchmark design X*. You can compute D-efficiency for any of the three D-errors
![\[ \textrm{D\mathrm{-}eff}(\textbf{X},\ \textbf{X}^\ast)=\frac{D(\textbf{X}^\ast)}{D(\textbf{X})} \]](data:image/svg+xml;nitro-empty-id=NjQ0Ojk2OQ==-1;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTgzIDQyIiB3aWR0aD0iMTgzIiBoZWlnaHQ9IjQyIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg== "Rendered by QuickLaTeX.com")
If the benchmark design has a lower D-error, the D-efficiency has a range from 0 to 1, and it will be close to 1 if the design is close to the benchmark.
Ready to find out more about D-Errors and assessing the quality of your design? Check out the Displayr blog.
