# Reversed Naive Bayes - likelihood and parameter estimation

What happens if we flip the arrows in a Naive Bayes classifier? To clarify - from what I have found naive Bayes is defined for the following network structure:

I'm interested to understand what happens if instead from y->x the child will be y. As shown in the following:

I'm not entirely sure if we get the same results as a regular naive Bayes classifier (intuitively we don't), and if not how to estimate both the likelihood of the network and the parameters?

For further clarification: lets say there is a training set of size n; each data point consisting of (x,y) where x is a vector of size m of binary values and y is the class and is also binary. E.g. (0,1,0,1,1) where the last index is the class. I'm trying to figure out the likelihood of the network (where y is the child) and how to estimate the parameters in the case of y being the child.

• This depends on how you interpret this. If you are looking at these pictures as a Directed Acyclic Graph (DAG) such as in a Bayesian Network, then the top picture depicts $X_1, X_2, X_3 | Y$ while the bottom shows $Y | X_1, X_2, X_3$. However, if it's not a DAG, then both are equivalent. – Jon Dec 11 '17 at 22:14

To elaborate on my comment, the first image depicts conditional dependence of $X_i | Y$ for $i \in 1,...,m$. Therefore, your likelihood for this model would be
\begin{aligned} P(Y = y) \times P(X_1 | Y = y) \times ... \times P(X_m | Y = y) \end{aligned}
The second image (bottom) depicts $Y | X_i$, therefore the likelihood would be
$$P(X_1) \times P(X_2) \times ... \times P(Y | X_1, X_2, ..., X_m) \\ \ = P(X_1) \times P(X_2) \times ... \times P(Y | X_1) \times ... \times P(Y | X_m)$$