# What does Determinant of Covariance Matrix give?

I am representing my 3d data in convariance matrix. I just want to know what the determinant of Convariance Matrix gives. If the determinant is positive, zero, negative, high positive, high negative. What does it mean or represent?

Thanks

EDIT:

Covariance is being used to represent variance for 3d coordiantes that I have. If my covariance matrix A determinant is +100, and the other other covariance matrix B determinant is +5. Which of these values show if the variance is more or not. Which value tells that datapoints are more dispersed. Which value shows that readings are further away from mean.

## 2 Answers

I would like to point out that there is a connection between the determinant of the covariance matrix of (Gaussian distributed) data points and the differential entropy of the distribution.

To put it in other words: Let's say you have a (large) set of points from which you assume it is Gaussian distributed. If you compute the determinant of the sample covariance matrix then you measure (indirectly) the differential entropy of the distribution up to constant factors and a logarithm. See, e.g, Multivariate normal distribution.

The differential entropy of a Gaussian density is defined as:

$$H[p] = \frac{k}{2}(1 + \ln(2\pi)) + \frac{1}{2} \ln \vert \Sigma \vert\;,$$

where $k$ is the dimensionality of your space, i.e., in your case $k=3$.

I think $\Sigma$ is positive semi-definite, which means $\vert \Sigma \vert \geq 0$. At least I have nerver seen $\vert \Sigma \vert < 0$.

The larger $\vert \Sigma \vert$, the more are your data points dispersed. If $\vert \Sigma \vert = 0$, it means that your data ponts do not 'occupy the whole space', meaning that they lie, e.g., on a line or a plane within $\mathbb{R}^3$. Somewhere I have read, that $\vert \Sigma \vert$ is also called generalized variance. Alexander Vigodner is right, it captures the volume of your data cloud.

Since a sample covariance matrix is defined somewhat like: $$\Sigma = \frac{1}{N-1} \sum_{i=1}^N (\vec{x}_i - \vec{\mu})(\vec{x}_i - \vec{\mu})^T\;$$ it follows, that you do not capture any information about the mean. You can verify that easily by adding some large constant vectorial shift to your data; $\vert \Sigma \vert$ should not change.

I don't want to go to much into detail, but there is also a connection to PCA. Since the eigenvalues $\lambda_1, \lambda_2, \lambda_3$ of $\Sigma$ correspond to the variances along the principal component axis of your data points, $\vert \Sigma \vert$ captures their product, because by definition the determinant of a matrix is equal to the product of its eigenvalues.

Note that the largest eigenvalue corresponds to the maximal variance w.r.t. to your data (direction given by the corresponding eigenvector, see PCA).

• What does it mean when H[p] is negative? – scottlittle Jan 28 '18 at 3:01

It cannot be negative, since the covariance matrix is positively (not necessary strictly) defined. So all it's eigenvalues are not negative and the determinant is product of these eigenvalues. It defines (square root of this) in certain sense the volume of n (3 in your case) dimensional $\sigma$-cube. It is analog $\sigma$ for 1 dimensional case.
Notice that mulitvarite normal distribution is defined as $$f_{\mathbf x}(x_1,\ldots,x_k) = \frac{1}{\sqrt{(2\pi)^k|\boldsymbol\Sigma|}} \exp\left(-\frac{1}{2}({\mathbf x}-{\boldsymbol\mu})^T{\boldsymbol\Sigma}^{-1}({\mathbf x}-{\boldsymbol\mu}) \right),$$ Here $|\Sigma|$ is determinant of $\Sigma$.

• Sorry but your answer does not help me. Covariance is being used to represent variance for 3d coordiantes that I have. If my covariance matrix A determinant is +100, and the other other covariance matrix B determinant is +5. Which of these values show if the variance is more or not. Which value tells that datapoints are more dispersed. Which value shows that readings are further away from mean. – orange14 Aug 6 '14 at 21:21
• Determinant is not so good characteristic for this. Use eigenvalues. – Alexander Vigodner Aug 6 '14 at 21:28
• Assume you have in 3 dimensional space eigenvalues $\lambda_1=\lambda_2=\lambda_3=1$ and in the second case $\lambda_1=1$, $\lambda_2=100$, $\lambda_3=0.01$. In both cases the determinant will be 1 but these two systems are completely different. This is why the comparing of determinants is not a good idea unless you have additional properties of your matrix $A$ $B$ that can help. – Alexander Vigodner Aug 6 '14 at 22:12
• so with eigen values what can I deduce. Which of these eigen values show if the variance is more or not. Which eigen value tells that data points are more dispersed. Which eigen value shows that readings are further away from mean – orange14 Aug 6 '14 at 22:49
• As tmp explained eigenvalues define variances along eigenvectors or principal component. – Alexander Vigodner Aug 7 '14 at 0:19