Question: What conclusion do they get after calculating the average number of known relations between topics’ elements?
Evidence: To estimate the quality of topic models in a real task, we chose Islam informational portal "Golos Islama" (Islam Voice) (in Russian). This portal contains both news articles related to Islam and articles discussing Islam basics. We supposed that the thematic analysis of this specialized site can be significantly improved with domain-specific knowledge described in the thesaurus form. We extracted the site contents using Open Web Spider and obtained 26,839 pages.
To combine knowledge with a topic model, we used RuThes thesaurus together with the additional block of the Islam thesaurus. The Islam thesaurus contains more than 5 thousand Islam-related terms including single words and expressions.
For each combined model, we ran two experiments with 100 topics and with 200 topics. The generated topics were evaluated by two linguists, who had previously worked on the Islam thesaurus. The evaluation task was formulated as follows: the experts should read the top elements of the generated topics and try to formulate labels of these topics. The labels should be different for each topic in the set generated with a specific model. The experts should also assign scores to the topics' labels:
Then we can sum up all the scores for each model under consideration and compare the total scores in value. Thus, maximum values of the topic score are 200 for a 100-topic model and 400 for a 200-topic model. In this experiment we do not measure inter-annotator agreement for each topic, but try to get expert's general impression.
Due to the complicated character of the Islam portal contents for automatic extraction (numerous words and names difficult for Russian morphological analyzers), we did not use automatic extraction of multiword expressions and exploited only phrases described in RuThes or in the Islam Thesaurus. We added thesaurus phrases in two ways: most frequent 1000 phrases (as in BIBREF14 , BIBREF16 ) and phrases with frequency more than 10 (More10phrases): the number of such phrases is 9351.
The results of the evaluation are shown in Table 5. The table contains the overall expert scores for a topic model (Score), kernel uniqueness as in the previous section (KernU), perplexity (Prpl). Also for each model kernels, we calculated the average number of known relations between topics’s elements: thesaurus relations (synonyms and direct relations between concepts) and component-based relations between phrases (Relc).
It can be seen that if we add phrases without accounting component similarity (Runs 2, 3), the quality of topics decreases: the more phrases are added, the more the quality degrades. The human scores also confirm this fact. But if the similarity between phrase components is considered then the quality of topics significantly improves and becomes better than for unigram models (Runs 4, 5). All measures are better. Relational coherence between kernel elements also grows. The number of added phrases is not very essential.
Adding unary synonyms decreases the quality of the models (Run 6) according to human scores. But all other measures behave differently: kernel uniqueness is high, perplexity decreases, relational coherence grows. The problem of this model is in that non-topical, general words are grouped together, reinforce one another but do not look as related to any topic. Adding all thesaurus relations is not very beneficial (Runs 7, 8). If we consider all relations except hyponyms, the human scores are better for corresponding runs (Runs 9, 10). Relational coherence in topics’ kernels achieves very high values: the quarter of all elements have some relations between each other, but it does not help to improve topics. The explanation is the same: general words can be grouped together.
At last, we removed General Lexicon concepts from the RuThes data, which are top-level, non-thematic concepts that can be met in arbitrary domains BIBREF19 and considered all-relations and without-hyponyms variants (Runs 11, 12). These last variants achieved maximal human scores because they add thematic knowledge and avoid general knowledge, which can distort topics. Kernel uniqueness is also maximal.
Table 6 shows similar topics obtained with the unigram, phrase-enriched (Run 5) and the thesaurus-enriched topic model (Run 12). The Run-5 model adds thesaurus phrases with frequency more than 10 and accounts for the component similarity between phrases. The Run-12 model accounts both component relations and hypernym thesaurus relations. All topics are of high quality, quite understandable. The experts evaluated them with the same high scores.
Phrase-enriched and thesaurus-enriched topics convey the content using both single words and phrases. It can be seen that phrase-enriched topics contain more phrases. Sometimes the phrases can create not very convincing relations such as Russian church - Russian language. It is explainable but does not seem much topical in this case.
The thesaurus topics seem to convey the contents in the most concentrated way. In the Syrian topic general word country is absent; instead of UN (United Nations), it contains word rebel, which is closer to the Syrian situation. In the Orthodox church topic, the unigram variant contains extra word year, relations of words Moscow and Kirill to other words in the topic can be inferred only from the encyclopedic knowledge.
Also for each model kernels, we calculated the average number of known relations between topics’s elements: thesaurus relations (synonyms and direct relations between concepts) and component-based relations between phrases (Relc). It can be seen that if we add phrases without accounting component similarity (Runs 2, 3), the quality of topics decreases: the more phrases are added, the more the quality degrades.
Answer: If phrases are added without accounting component similarity (Runs 2, 3), the quality of topics decreases: the more phrases are added, the more the quality degrades.