# Interpreting the result of topic modelling using Latent Dirichlet Allocation

I have an unpublished corpus consisting of 135 texts representing one year each. I do topic modelling (using Mallet) and then inspect the topic distribution over time. The overall picture looks good: In the contribution table, showing the topic-composition of the single texts, every dozen years a new topic dominates and the topics have nice bell shaped curves in a topic percentage-time diagram with peak percentages in the range of 50 to 70.

However, the oldest and the most recent years behave differently: There the respective topics rise to really high values (above 90%, in one case even above 99%). But: There is no reason to think that topics should behave differently just because there are no older or newer data available.

My question is: How are these artefacts explained? Are there measures to mitigate the artefacts?

EDIT: I used the full corpus for training the topics. The topics are kind of buzzwords typically for the respective years.

I used a "small" number of topics (10) and every topic is for some time the dominating topic in the corpus (with more than 50% contribution for some years).

Intuitively spoken, Latent Dirichlet Allocation cannot see the topics from the past or the future (outside of the corpus) that should give some 10--20% contribution to the topic mix at the boundaries.

EDIT2: I used MALLET's default values for the hyperparameters, i.e., alpha=50.0 beta=0.01 gamma=0.01 delta=0.03 delta1=0.2 delata2=1000.0. I also used the defaults on --use-symmetric-alpha (false) and --use-ngrams (false)

• Can you say more about what this diagram represents? LDA represents each document as a distribution over topics, and it does seem odd that the newest and oldest documents would be so topically sparse. Were the topics trained from the entire corpus? Jun 12 '15 at 13:55
• @SeanEaster: See my EDIT
– user79309
Jun 13 '15 at 13:04
• The high values that you reference, are they the maximum among each document's posterior $\alpha$-parameter, or something else? Jun 14 '15 at 16:53
• @SeanEaster The high percentages are coming from the contribution table showing the contributions of the topics to the texts in the corpus. So, the oldest text is almost completely consisting of a single topic with no admixture of other topics.
– user79309
Jun 15 '15 at 8:34
• I'm not familiar with Mallet, but that sounds very much like the topic distribution for each document. If terms go in and out of style over the life of the corpus, it's sensible that certain topics would collect terms only used in the earliest and latest documents. What's strange is that these documents have so few words from the middle topics. As you allude to in your second edit, the hyper-parameters are related to this. For more on this relationship, I'd consult David Blei's lecture here. Jun 15 '15 at 13:21