# [Topic-models] Questions regarding hLDA

Rachit Arora rachitisthere at gmail.com
Sat May 17 05:41:08 EDT 2008

Hi,
I have some questions regarding hLDA model. I am sorry if they have been
answered before or are too naive

1. Can two topics in different dis-joint paths of the hLDA tree be same.
Example let us say there are 3 documents, 1st one has topics {A,B,C,D} , 2nd
one has topics {A,B,C,E} and 3rd one has topics {A,B,F,E}
Thus all 3 paths are joint along topics {A,B}, the 3rd document splits here
and the 1st two documents continue to the node C on the common path.
But the 3rd document and the 2nd document have a common topic E. Thus is the
topic E of document 3 the same as the topic E of document 2 or they both
have to be different ?

2. I couldn't get the point of generating a word from the root restaurant.
This is assuming that all the documents have at least 1 common topic, which
might not be the case ( if we are ignoring the stop-words )

3. The number of topics in the entire collection of the documents is K and
each document has L topics. The topics of the documents form a subset of the
total number of topics in the corpus. Thus K can range from L to M*L ( the
former case when all the documents have the same path and the latter when
all are dis-joint except at the root ). However the distribution of the
topics over the words from Dir(\beta) has K as a constant. Or is the
distribution of topics over words sampled from Dir(\beta) everytime we
sample the path for a document, since it might increase ( or decrease ) the
total number of topics in the corpus i.e. K

I am confused over these points in the paper and would be of great help if
someone could clear them

Thanks
- Rachit Arora

--
Rachit Arora
4th Year
Computer Science and Engineering