# Predictive posterior distribution with multivariate normal distribution

Suppose I have a multivariate normal ${\bf{Y}}|{\bf{\theta}} \sim {\bf{MVN}}(X {\bf{\beta}}, \sigma^{2}H(\phi))$ where ${\bf{Y}}$ is a set of observations ${\bf{Y}} = \{y({\bf{s}}_{1}),y({\bf{s}}_{2}),... ,y({\bf{s}}_{n})\}$ and $H(\phi)$ is a covariance matrix $H(\phi) = \rho({\bf{s}}_{i} - {\bf{s}}_{j};\phi)$. Now I want to predict a value $y_{0}$, so as it is usual, the predictive posterior distribution is

$$p(y_{0}|{\bf{y}}, X, x_{0}) = \int p(y_{0}|{\bf{y}},{\bf{\theta}}, x_0)p({\bf{\theta}}|{\bf{y}}, X)d\theta$$

Usually, this is described as a product of $\text{Likelihood} \times \text{Posterior}$. However, in this case, the likelihood was initially expressed as a multivariate normal but I just want to predict the value $y({\bf{s}})$ at some point ${\bf{s}}_{0}$, so obviously I can't express a single point as a multivariate normal. What should I do?

As an alternative, I should be able to use a conditional distribution $y_{0} | \bf{Y}$ from a joint distribution $\left( y_{0}, {\bf{Y}} \right)$ in the following way:

$$\left( \begin{array}{ccc} Y_{1} \\ Y_{2} \end{array} \right) \sim N\left(\left( \begin{array}{ccc} \mu_{1} \\ \mu_{2} \end{array} \right), \left( \begin{array}{ccc} \Omega_{11} & \Omega_{12} \\ \Omega_{21} & \Omega_{22} \end{array} \right)\right)$$

The problem is that this in some way breaks with the idea of using $\text{Likelihood} \times \text{Posterior}$ for the predictive posterior distribution.

Thanks!

• I am not sure that this question is answerable as written. I don't see how you can predict a single point when you didn't pick a point (or time frame) to start with. Moreover, since you have a multivariate normal, you don't even have a point, but rather a matrix. By taking expectations, you can update the mean using the equation you provided. If you just want to predict a single point, I think you have to articulate further what it is that you want to do, i.e. the marginal mean, the conditional mean, etc. – rocinante Sep 12 '14 at 18:20
• I'm not sure what you mean that I didn't pick a a point to start with. I have a whole set of observations $\bf{Y}$. Maybe, the confusion arises from the fact that I want to obtain the predictive posterior distribution for $y_{0}({\bf{s}})$ at some point $s_{0}$. I will update the question to make it clear. – Robert Smith Sep 12 '14 at 18:53

It sounds like you are trying to do inference with a Gaussian process. The correct approach is to use the conditional distribution, as in the second part of your question. The reason that you cannot use the Likelihood $\times$ Posterior approach is because $\theta$ is an infinite-dimensional object, representing the value of the process at all locations ${\bf s}$.

• Thank you for your answer. $\theta$ is an infinite-dimensional object? I have been using $\theta$ as parameters. Are yuu sure about that? By the way, I'm not necessarily using Gaussian processes. I know that the derivation is the same, but here the dimensions are finite, although I don't think there is a strong distinction when implementing Gaussian processes in practice. – Robert Smith Sep 13 '14 at 19:01
• I just proposed a very simple explanation of why a conditional distribution is needed. If you notice a mistake, please let me know. – Robert Smith Sep 13 '14 at 19:38

I think I got a very simple answer.

I overlooked the fact that the predictive posterior distribution is derived with a "conditional likelihood":

\begin{array} pp(y_{0}|{\bf{y}}) &= \int p(y_{0}, \theta| {\bf{y}})d\theta \\ & = \int p(y_{0}|{\theta, \bf{y}})p(\theta| {\bf{y}})d\theta \end{array}

where I omitted ${\bf{X}}$ and $x_{0}$ for simplicity. The full predictive posterior distribution is what I initially wrote in my question:

$$p(y_{0}|{\bf{y}}, X, x_{0}) = \int p(y_{0}|{\bf{y}},{\bf{\theta}}, x_0)p({\bf{\theta}}|{\bf{y}}, X)d\theta$$

Only when $y_{0}$ is independent of the previous observations ${\bf{y}}$, it is possible to set $p(y_{0}|{\bf{y}},{\bf{\theta}}, x_0) = p(y_{0}|{\bf{\theta}}, x_0)$ to obtain the regular form $$\text{Predictive} = \text{Likelihood} \times \text{Posterior}$$

$$p(y_{0}|{\bf{y}}, X, x_{0}) = \int p(y_{0}|{\bf{\theta}}, x_0)p({\bf{\theta}}|{\bf{y}}, X)d\theta$$