In information theory, dual total correlation,^[1] information rate,^[2] excess entropy,^[3][4] or binding information^[5] is one of several known non-negative generalizations of mutual information. While total correlation is bounded by the sum entropies of the n elements, the dual total correlation is bounded by the joint-entropy of the n elements. Although well behaved, dual total correlation has received much less attention than the total correlation. A measure known as "TSE-complexity" defines a continuum between the total correlation and dual total correlation.^[3]

For a set of n random variables ${\displaystyle \{X_{1},\ldots ,X_{n}\}}$, the dual total correlation ${\displaystyle D(X_{1},\ldots ,X_{n})}$ is given by

${\displaystyle D(X_{1},\ldots ,X_{n})=H\left(X_{1},\ldots ,X_{n}\right)-\sum _{i=1}^{n}H\left(X_{i}\mid X_{1},\ldots ,X_{i-1},X_{i+1},\ldots ,X_{n}\right),}$

where ${\displaystyle H(X_{1},\ldots ,X_{n})}$ is the joint entropy of the variable set ${\displaystyle \{X_{1},\ldots ,X_{n}\}}$ and ${\displaystyle H(X_{i}\mid \cdots )}$ is the conditional entropy of variable ${\displaystyle X_{i}}$, given the rest.

The dual total correlation normalized between [0,1] is simply the dual total correlation divided by its maximum value ${\displaystyle H(X_{1},\ldots ,X_{n})}$,

${\displaystyle ND(X_{1},\ldots ,X_{n})={\frac {D(X_{1},\ldots ,X_{n})}{H(X_{1},\ldots ,X_{n})}}.}$

Dual total correlation is non-negative and bounded above by the joint entropy ${\displaystyle H(X_{1},\ldots ,X_{n})}$.

${\displaystyle 0\leq D(X_{1},\ldots ,X_{n})\leq H(X_{1},\ldots ,X_{n}).}$

Secondly, Dual total correlation has a close relationship with total correlation, ${\displaystyle C(X_{1},\ldots ,X_{n})}$, and can be written in terms of differences between the total correlation of the whole, and all subsets of size ${\displaystyle N-1}$:^[6]

${\displaystyle D({\textbf {X}})=(N-1)C({\textbf {X}})-\sum _{i=1}^{N}C({\textbf {X}}^{-i})}$

where ${\displaystyle {\textbf {X}}=\{X_{1},\ldots ,X_{n}\}}$ and ${\displaystyle {\textbf {X}}^{-i}=\{X_{1},\ldots ,X_{i-1},X_{i+1},\ldots ,X_{n}\}}$

Furthermore, the total correlation and dual total correlation are related by the following bounds:

${\displaystyle {\frac {C(X_{1},\ldots ,X_{n})}{n-1}}\leq D(X_{1},\ldots ,X_{n})\leq (n-1)\;C(X_{1},\ldots ,X_{n}).}$

Finally, the difference between the total correlation and the dual total correlation defines a novel measure of higher-order information-sharing: the O-information:^[7]

${\displaystyle \Omega ({\textbf {X}})=C({\textbf {X}})-D({\textbf {X}})}$.

The O-information (first introduced as the "enigmatic information" by James and Crutchfield^[8] is a signed measure that quantifies the extent to which the information in a multivariate random variable is dominated by synergistic interactions (in which case ${\displaystyle \Omega ({\textbf {X}})<0}$) or redundant interactions (in which case ${\displaystyle \Omega ({\textbf {X}})>0}$.

Han (1978) originally defined the dual total correlation as,

${\displaystyle {\begin{aligned}&D(X_{1},\ldots ,X_{n})\\[10pt]\equiv {}&\left[\sum _{i=1}^{n}H(X_{1},\ldots ,X_{i-1},X_{i+1},\ldots ,X_{n})\right]-(n-1)\;H(X_{1},\ldots ,X_{n})\;.\end{aligned}}}$

However Abdallah and Plumbley (2010) showed its equivalence to the easier-to-understand form of the joint entropy minus the sum of conditional entropies via the following:

${\displaystyle {\begin{aligned}&D(X_{1},\ldots ,X_{n})\\[10pt]\equiv {}&\left[\sum _{i=1}^{n}H(X_{1},\ldots ,X_{i-1},X_{i+1},\ldots ,X_{n})\right]-(n-1)\;H(X_{1},\ldots ,X_{n})\\={}&\left[\sum _{i=1}^{n}H(X_{1},\ldots ,X_{i-1},X_{i+1},\ldots ,X_{n})\right]+(1-n)\;H(X_{1},\ldots ,X_{n})\\={}&H(X_{1},\ldots ,X_{n})+\left[\sum _{i=1}^{n}H(X_{1},\ldots ,X_{i-1},X_{i+1},\ldots ,X_{n})-H(X_{1},\ldots ,X_{n})\right]\\={}&H\left(X_{1},\ldots ,X_{n}\right)-\sum _{i=1}^{n}H\left(X_{i}\mid X_{1},\ldots ,X_{i-1},X_{i+1},\ldots ,X_{n}\right)\;.\end{aligned}}}$

• Interaction information
• Mutual information
• Total correlation