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Hi Yan, Good work. Your hypothesis is stated and motivated well. But few of the claims you make lack justifications from the work you presented.
1. NB has better performance in our experiments. However, our first guess is that DBN should be better. The reason might be the size of DBN is too small.
You mean adding more layers would always help? Why you feel that there is no saturation? And what are some std practice then to decide the number of layers. Is there a principled way to set this?
4. Data that contains more spams lead to better performance.
True as it sees more thing to learn. What would be more interesting if spam/(not spam) ratio has any impact, to be able to generalize the way you stated
5. NB is more suitable for serving as a prototype for spam detection. While DBN can achieve better performance in larger system with more data and computational power.
You can only claim this when you can actually see this happening, but as of now you are just going by gut feeling :-). Moreover no study on running/training time is performed. Which leaves the validity un-justified again.