Using Conditional Random Fields to Extract Contexts and Answers of Questions from Online Forums
Shilin Ding   Gao Cong§  Chin-Yew Lin Xiaoyan Zhu Department of Computer Science and Technology, Tsinghua University, Beijing, China §Department of Computer Science, Aalborg University, Denmark Microsoft Research Asia, Beijing, China dingsl@gmail.com gaocong@cs.aau.dk cyl@microsoft.com zxy-dcs@tsinghua.edu.cn Abstract
Online forum discussions often contain vast amounts of questions that are the focuses of discussions. Extracting contexts and answers together with the questions will yield not only a coherent forum summary but also a valuable QA knowledge base. In this paper, we propose a general framework based on Conditional Random Fields (CRFs) to detect the contexts and answers of questions from forum threads. We improve the basic framework by Skip-chain CRFs and 2D CRFs to better accommodate the features of forums for better performance. Experimental results show that our techniques are very promising. <context id=1>S1: Hi I am looking for a pet friendly hotel in Hong Kong because all of my family is going there for vacation. S2: my family has 2 sons and a dog.</context> <question id=1>S3: Is there any recommended hotel near Sheung Wan or Tsing Sha Tsui?</question> <context id=2,3>S4: We also plan to go shopping in Causeway Bay.</context> <question id=2>S5: What's the traffic situation around those commercial areas?</question> <question id=3>S6: Is it necessary to take a taxi?</question>. S7: Any information would be appreciated. <answer qid=1>S8: The Comfort Lodge near Kowloon Park allows pet as I know, and usually fits well within normal budget. S9: It is also conveniently located, nearby the Kowloon railway station and subway.</answer> <answer qid=2,3> S10: It's very crowd in those areas, so I recommend MTR in Causeway Bay because it is cheap to take you around </answer> Figure 1: An example thread with question-contextanswer annotated

1 Introduction
Forums are web virtual spaces where people can ask questions, answer questions and participate in discussions. The availability of vast amounts of thread discussions in forums has promoted increasing interests in knowledge acquisition and summarization for forum threads. Forum thread usually consists of an initiating post and a number of reply posts. The initiating post usually contains several questions and the reply posts usually contain answers to the questions and perhaps new questions. Forum participants are not physically co-present, and thus reply may not happen immediately after questions are posted. The asynchronous nature and multiparticipants make multiple questions and answers
 This work was done when Shilin Ding was a visiting student at the Microsoft Research Asia  This work was done when Gao Cong worked as a researcher at the Microsoft Research Asia.

interweaved together, which makes it more difficult to summarize. In this paper, we address the problem of detecting the contexts and answers from forum threads for the questions identified in the same threads. Figure 1 gives an example of a forum thread with questions, contexts and answers annotated. It contains three question sentences, S3, S5 and S6. Sentences S1 and S2 are contexts of question 1 (S3). Sentence S4 is the context of questions 2 and 3, but not 1. Sentence S8 is the answer to question 3. (S4-S5-S10) is one example of question-context-answer triple that we want to detect in the thread. As shown in the example, a forum question usually requires contextual information to provide background or constraints.

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Proceedings of ACL-08: HLT, pages 710­718, Columbus, Ohio, USA, June 2008. c 2008 Association for Computational Linguistics


Moreover, it sometimes needs contextual information to provide explicit link to its answers. For example, S8 is an answer of question 1, but they cannot be linked with any common word. Instead, S8 shares word pet with S1, which is a context of question 1, and thus S8 could be linked with question 1 through S1. We call contextual information the context of a question in this paper. A summary of forum threads in the form of question-context-answer can not only highlight the main content, but also provide a user-friendly organization of threads, which will make the access to forum information easier. Another motivation of detecting contexts and answers of the questions in forum threads is that it could be used to enrich the knowledge base of community-based question and answering (CQA) services such as Live QnA and Yahoo! Answers, where context is comparable with the question description while question corresponds to the question title. For example, there were about 700,000 questions in the Yahoo! Answers travel category as of January 2008. We extracted about 3,000,000 travel related questions from six online travel forums. One would expect that a CQA service with large QA data will attract more users to the service. To enrich the knowledge base, not only the answers, but also the contexts are critical; otherwise the answer to a question such as How much is the taxi would be useless without context in the database. However, it is challenging to detecting contexts and answers for questions in forum threads. We assume the questions have been identified in a forum thread using the approach in (Cong et al., 2008). Although identifying questions in a forum thread is also nontrivial, it is beyond the focus of this paper. First, detecting contexts of a question is important and non-trivial. We found that 74% of questions in our corpus, which contain 1,064 questions from 579 forum threads about travel, need contexts. However, relative position information is far from adequate to solve the problem. For example, in our corpus 63% of sentences preceding questions are contexts and they only represent 34% of all correct contexts. To effectively detect contexts, the dependency between sentences is important. For example in Figure 1, both S1 and S2 are contexts of question 1. S1 could be labeled as context based on word similarity, but it

is not easy to link S2 with the question directly. S1 and S2 are linked by the common word family, and thus S2 can be linked with question 1 through S1. The challenge here is how to model and utilize the dependency for context detection. Second, it is difficult to link answers with questions. In forums, multiple questions and answers can be discussed in parallel and are interweaved together while the reply relationship between posts is usually unavailable. To detect answers, we need to handle two kinds of dependencies. One is the dependency relationship between contexts and answers, which should be leveraged especially when questions alone do not provide sufficient information to find answers; the other is the dependency between answer candidates (similar to sentence dependency described above). The challenge is how to model and utilize these two kinds of dependencies. In this paper we propose a novel approach for detecting contexts and answers of the questions in forum threads. To our knowledge this is the first work on this. We make the following contributions: First, we employ Linear Conditional Random Fields (CRFs) to identify contexts and answers, which can capture the relationships between contiguous sentences. Second, we also found that context is very important for answer detection. To capture the dependency between contexts and answers, we introduce Skip-chain CRF model for answer detection. We also extend the basic model to 2D CRFs to model dependency between contiguous questions in a forum thread for context and answer identification. Finally, we conducted experiments on forum data. Experimental results show that 1) Linear CRFs outperform SVM and decision tree in both context and answer detection; 2) Skip-chain CRFs outperform Linear CRFs for answer finding, which demonstrates that context improves answer finding; 3) 2D CRF model improves the performance of Linear CRFs and the combination of 2D CRFs and Skipchain CRFs achieves better performance for context detection. The rest of this paper is organized as follows: The next section discusses related work. Section 3 presents the proposed techniques. We evaluate our techniques in Section 4. Section 5 concludes this paper and discusses future work.

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2 Related Work
There is some research on summarizing discussion threads and emails. Zhou and Hovy (2005) segmented internet relay chat, clustered segments into subtopics, and identified responding segments of the first segment in each sub-topic by assuming the first segment to be focus. In (Nenkova and Bagga, 2003; Wan and McKeown, 2004; Rambow et al., 2004), email summaries were organized by extracting overview sentences as discussion issues. Carenini et al (2007) leveraged both quotation relation and clue words for email summarization. In contrast, given a forum thread, we extract questions, their contexts, and their answers as summaries. Shrestha and McKeown (2004)'s work on email summarization is closer to our work. They used RIPPER as a classifier to detect interrogative questions and their answers and used the resulting question and answer pairs as summaries. However, it did not consider contexts of questions and dependency between answer sentences. We also note the existing work on extracting knowledge from discussion threads. Huang et al.(2007) used SVM to extract input-reply pairs from forums for chatbot knowledge. Feng et al. (2006a) used cosine similarity to match students' query with reply posts for discussion-bot. Feng et al. (2006b) identified the most important message in online classroom discussion board. Our problem is quite different from the above work. Detecting context for question in forums is related to the context detection problem raised in the QA roadmap paper commissioned by ARDA (Burger et al., 2006). To our knowledge, none of the previous work addresses the problem of context detection. The method of finding follow-up questions (Yang et al., 2006) from TREC context track could be adapted for context detection. However, the followup relationship is limited between questions while context is not. In our other work (Cong et al., 2008), we proposed a supervised approach for question detection and an unsupervised approach for answer detection without considering context detection. Extensive research has been done in questionanswering, e.g. (Berger et al., 2000; Jeon et al., 2005; Cui et al., 2005; Harabagiu and Hickl, 2006; Dang et al., 2007). They mainly focus on con-

structing answer for certain types of question from a large document collection, and usually apply sophisticated linguistic analysis to both questions and the documents in the collection. Soricut and Brill (2006) used statistical translation model to find the appropriate answers from their QA pair collections from FAQ pages for the posted question. In our scenario, we not only need to find answers for various types of questions in forum threads but also their contexts.

3 Context and Answer Detection
A question is a linguistic expression used by a questioner to request information in the form of an answer. The sentence containing request focus is called question. Context are the sentences containing constraints or background information to the question, while answer are that provide solutions. In this paper, we use sentences as the detection segment though it is applicable to other kinds of segments. Given a thread and a set of m detected questions {Qi }m 1 , our task is to find the contexts and ani= swers for each question. We first discuss using Linear CRFs for context and answer detection, and then extend the basic framework to Skip-chain CRFs and 2D CRFs to better model our problem. Finally, we will briefly introduce CRF models and the features that we used for CRF model. 3.1 Using Linear CRFs For ease of presentation, we focus on detecting contexts using Linear CRFs. The model could be easily extended to answer detection. Context detection. As discussed in Introduction that context detection cannot be trivially solved by position information (See Section 4.2 for details), and dependency between sentences is important for context detection. Recall that in Figure 1, S2 could be labeled as context of Q1 if we consider the dependency between S2 and S1, and that between S1 and Q1, while it is difficult to establish connection between S2 and Q1 without S1. Table 1 shows that the correlation between the labels of contiguous sentences is significant. In other words, when a sentence Yt 's previous Yt-1 is not a context (Yt-1 = C) then it is very likely that Yt (i.e. Yt = C) is also not a context. It is clear that the candidate contexts are not independent and there are strong dependency rela-

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Contiguous sentences yt-1 = C yt-1 = C

yt = C 901 1,081

yt = C 1,081 47,190

Table 1: Contingency table(2 = 9,386,p-value<0.001)

tionships between contiguous sentences in a thread. Therefore, a desirable model should be able to capture the dependency. The context detection can be modeled as a classification problem. Traditional classification tools, e.g. SVM, can be employed, where each pair of question and candidate context will be treated as an instance. However, they cannot capture the dependency relationship between sentences. To this end, we proposed a general framework to detect contexts and answers based on Conditional Random Fields (Lafferty et al., 2001) (CRFs) which are able to model the sequential dependencies between contiguous nodes. A CRF is an undirected graphical model G of the conditional distribution P (Y|X). Y are the random variables over the labels of the nodes that are globally conditioned on X, which are the random variables of the observations. (See Section 3.4 for more about CRFs) Linear CRF model has been successfully applied in NLP and text mining tasks (McCallum and Li, 2003; Sha and Pereira, 2003). However, our problem cannot be modeled with Linear CRFs in the same way as other NLP tasks, where one node has a unique label. In our problem, each node (sentence) might have multiple labels since one sentence could be the context of multiple questions in a thread. Thus, it is difficult to find a solution to tag context sentences for all questions in a thread in single pass. Here we assume that questions in a given thread are independent and are found, and then we can label a thread with m questions one-by-one in mpasses. In each pass, one question Qi is selected as focus and each other sentence in the thread will be labeled as context C of Qi or not using Linear CRF model. The graphical representations of Linear CRFs is shown in Figure2(a). The linear-chain edges can capture the dependency between two contiguous nodes. The observation sequence x = <x1 , x2 ,...,xt >, where t is the number of sentences in a thread, represents predictors (to be described in Section 3.5), and the tag sequence y=<y1 ,...,yt >, where

yi  {C, P }, determines whether a sentence is plain text P or context C of question Qi . Answer detection. Answers usually appear in the posts after the post containing the question. There are also strong dependencies between contiguous answer segments. Thus, position and similarity information alone are not adequate here. To cope with the dependency between contiguous answer segments, Linear CRFs model are employed as in context detection. 3.2 Leveraging Context for Answer Detection Using Skip-chain CRFs We observed in our corpus 74% questions lack constraints or background information which are very useful to link question and answers as discussed in Introduction. Therefore, contexts should be leveraged to detect answers. The Linear CRF model can capture the dependency between contiguous sentences. However, it cannot capture the long distance dependency between contexts and answers. One straightforward method of leveraging context is to detect contexts and answers in two phases, i.e. to first identify contexts, and then label answers using both the context and question information (e.g. the similarity between context and answer can be used as features in CRFs). The two-phase procedure, however, still cannot capture the non-local dependency between contexts and answers in a thread. To model the long distance dependency between contexts and answers, we will use Skip-chain CRF model to detect context and answer together. Skipchain CRF model is applied for entity extraction and meeting summarization (Sutton and McCallum, 2006; Galley, 2006). The graphical representation of a Skip-chain CRF given in Figure2(b) consists of two types of edges: linear-chain (yt-1 to yt ) and skip-chain edges (yi to yj ). Ideally, the skip-chain edges will establish the connection between candidate pairs with high probability of being context and answer of a question. To introduce skip-chain edges between any pairs of non-contiguous sentences will be computationally expensive, and also introduce noise. To make the cardinality and number of cliques in the graph manageable and also eliminate noisy edges, we would like to generate edges only for sentence pairs with high possibility of being context and answer. This is

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(a) Linear CRFs

(b) Skip-chain CRFs

(c) 2D CRFs

Figure 2: CRF Models

Skip-Chain yu = C yu = C

yv = A 4,105 3,744

yv = A 5,314 9,740

Table 2: Contingence table(2 =615.8,p-value < 0.001)

achieved as follows. Given a question Qi in post Pj of a thread with n posts, its contexts usually occur within post Pj or before Pj while answers appear in the posts after Pj . We will establish an edge between each candidate answer v and one condidate context in {Pk }j =1 such that they have the highest possibilk ity of being a context-answer pair of question Qi : u = argmax sim(xu , Qi ).sim(xv , {xu , Qi })
u{Pk }j =1 k

here, we use the product of sim(xu , Qi ) and sim(xv , {xu , Qi } to estimate the possibility of being a context-answer pair for (u, v ) , where sim(ˇ, ˇ) is the semantic similarity calculated on WordNet as described in Section 3.5. Table 2 shows that yu and yv in the skip chain generated by our heuristics influence each other significantly. Skip-chain CRFs improve the performance of answer detection due to the introduced skip-chain edges that represent the joint probability conditioned on the question, which is exploited by skip-chain feature function: f (yu , yv , Qi , x). 3.3 Using 2D CRF Model Both Linear CRFs and Skip-chain CRFs label the contexts and answers for each question in separate passes by assuming that questions in a thread are independent. Actually the assumption does not hold in many cases. Let us look at an example. As in Figure 1, sentence S10 is an answer for both question 7 Q2 and Q3. S10 could be recognized as the answer of Q2 due to the shared word areas and Causeway

bay (in Q2's context, S4), but there is no direct relation between Q3 and S10. To label S10, we need consider the dependency relation between Q2 and Q3. In other words, the question-answer relation between Q3 and S10 can be captured by a joint modeling of the dependency among S10, Q2 and Q3. The labels of the same sentence for two contiguous questions in a thread would be conditioned on the dependency relationship between the questions. Such a dependency cannot be captured by both Linear CRFs and Skip-chain CRFs. To capture the dependency between the contiguous questions, we employ 2D CRFs to help context and answer detection. 2D CRF model is used in (Zhu et al., 2005) to model the neighborhood dependency in blocks within a web page. As shown in Figure2(c), 2D CRF models the labeling task for all questions in a thread. For each thread, there are m rows in the grid, where the ith row corresponds to one pass of Linear CRF model (or Skip-chain model) which labels contexts and answers for question Qi . The vertical edges in the figure represent the joint probability conditioned on the contiguous questions, which will be exploited by 2D feature function: f (yi,j , yi+1,j , Qi , Qi+1 , x). Thus, the information generated in single CRF chain could be propagated over the whole grid. In this way, context and answer detection for all questions in the thread could be modeled together. 3.4 Conditional Random Fields (CRFs) The Linear, Skip-Chain and 2D CRFs can be generalized as pairwise CRFs, which have two kinds of cliques in graph G: 1) node yt and 2) edge (yu , yv ). The joint probability is defined as:
p(y|x)= k k 1 exp k fk (yt , x)+ ľk gk (yu , yv , x) Z (x)
,t ,t

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where Z (x) is the normalization factor, fk is the feature on nodes, gk is on edges between u and v , and k and ľk are parameters. Linear CRFs are based on the first order Markov assumption that the contiguous nodes are dependent. The pairwise edges in Skip-chain CRFs represent the long distance dependency between the skipped nodes, while the ones in 2D CRFs represent the dependency between the neighboring nodes. Inference and Parameter Estimation. For Linear CRFs, dynamic programming is used to compute the maximum a posteriori (MAP) of y given x. However, for more complicated graphs with cycles, exact inference needs the junction tree representation of the original graph and the algorithm is exponential to the treewidth. For fast inference, loopy Belief Propagation (Pearl, 1988) is implemented. Given the training Data D = {x(i) , y(i) }n 1 , the i= parameter estimation is to determine the parameters based on maximizing the log-likelihood L = n (i) (i) i=1 log p(y |x ). In Linear CRF model, dynamic programming and L-BFGS (limited memory Broyden-Fletcher-Goldfarb-Shanno) can be used to optimize objective function L , while for complicated CRFs, Loopy BP are used instead to calculate the marginal probability. 3.5 Features used in CRF models The main features used in Linear CRF models for context detection are listed in Table 3. The similarity feature is to capture the word similarity and semantic similarity between candidate contexts and answers. The word similarity is based on cosine similarity of TF/IDF weighted vectors. The semantic similarity between words is computed based on Wu and Palmer's measure (Wu and Palmer, 1994) using WordNet (Fellbaum, 1998).1 The similarity between contiguous sentences will be used to capture the dependency for CRFs. In addition, to bridge the lexical gaps between question and context, we learned top-3 context terms for each question term from 300,000 question-description pairs obtained from Yahoo! Answers using mutual information (Berger et al., 2000) ( question description in Yahoo! Answers is comparable to contexts in foThe semantic similarity between sentences is calculated as in (Yang et al., 2006).
1

Similarity features: ˇ Cosine similarity with the question ˇ Similarity with the question using WordNet ˇ Cosine similarity between contiguous sentences ˇ Similarity between contiguous sentences using WordNet ˇ Cosine similarity with the expanded question using the lexical matching words Structural features: ˇ The relative position to current question ˇ Is its author the same with that of the question? ˇ Is it in the same paragraph with its previous sentence? Discourse and lexical features: ˇ The number of Pronouns in the question ˇ The presence of fillers, fluency devices (e.g. "uh", "ok") ˇ The presence of acknowledgment tokens ˇ The number of non-stopwords ˇ Whether the question has a noun or not? ˇ Whether the question has a verb or not?

Table 3: Features for Linear CRFs. Unless otherwise mentioned, we refer to features of the sentence whose label to be predicted

rums), and then use them to expand question and compute cosine similarity. The structural features of forums provide strong clues for contexts. For example, contexts of a question usually occur in the post containing the question or preceding posts. We extracted the discourse features from a question, such as the number of pronouns in the question. A more useful feature would be to find the entity in surrounding sentences referred by a pronoun. We tried GATE (Cunningham et al., 2002) for anaphora resolution of the pronouns in questions, but the performance became worse with the feature, which is probably due to the difficulty of anaphora resolution in forum discourse. We also observed that questions often need context if the question do not contain a noun or a verb. In addition, we use similarity features between skip-chain sentences for Skip-chain CRFs and similarity features between questions for 2D CRFs.

4 Experiments
4.1 Experimental setup Corpus. We obtained about 1 million threads from TripAdvisor forum; we randomly selected 591 threads and removed 22 threads which has more than 40 sentences and 6 questions; the remaining 579 forum threads form our corpus 2 . Each thread in our
TripAdvisor (http://www.tripadvisor.com/ForumHome) is one of the most popular travel forums; the list of 579 urls is
2

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Model SVM C4.5 L-CRF SVM C4.5 L-CRF

Prec(%) Rec(%) Context Detection 75.27 68.80 70.16 64.30 75.75 72.84 Answer Detection 73.31 47.35 65.36 46.55 63.92 58.74

F1 (%) 71.32 67.21 74.45 57.52 54.37 61.22

Prec(%) Rec(%) Context Detection Previous One 63.69 34.29 Previous All 43.48 76.41 Anwer Detection Following One 66.48 19.98 Following All 31.99 100

position

F1 (%) 44.58 55.42 30.72 48.48

Table 5: Using position information for detection

Table 4: Context and Answer Detection

corpus contains at least two posts and on average each thread consists of 3.87 posts. Two annotators were asked to tag questions, their contexts, and answers in each thread. The kappa statistic for identifying question is 0.96, for linking context and question given a question is 0.75, and for linking answer and question given a question is 0.69. We conducted experiments on both the union and intersection of the two annotated data. The experimental results on both data are qualitatively comparable. We only report results on union data due to space limitation. The union data contains 1,064 questions, 1,458 contexts and 3,534 answers. Metrics. We calculated precision, recall, and F1 -score for all tasks. All the experimental results are obtained through the average of 5 trials of 5-fold cross validation. 4.2 Experimental results Linear CRFs for Context and Answer Detection. This experiment is to evaluate Linear CRF model (Section 3.1) for context and answer detection by comparing with SVM and C4.5(Quinlan, 1993). For SVM, we use SVMlight (Joachims, 1999). We tried linear, polynomial and RBF kernels and report the results on polynomial kernel using default parameters since it performs the best in the experiment. SVM and C4.5 use the same set of features as Linear CRFs. As shown in Table 4, Linear CRF model outperforms SVM and C4.5 for both context and answer detection. The main reason for the improvement is that CRF models can capture the sequential dependency between segments in forums as discussed in Section 3.1.
given in http://homepages.inf.ed.ac.uk/gcong/acl08/; Removing the 22 long threads can greatly reduce the training and test time.

Context No context Prev. sentence Real context L-CRF+context

Prec(%) 63.92 61.41 63.54 65.51

Rec(%) 58.74 62.50 66.40 63.13

F1 (%) 61.22 61.84 64.94 64.06

Table 6: Contextual Information for Answer Detection. Prev. sentence uses one previous sentence of the current question as context. RealContext uses the context annotated by experts. L-CRF+context uses the context found by Linear CRFs

We next report a baseline of context detection using previous sentences in the same post with its question since contexts often occur in the question post or preceding posts. Similarly, we report a baseline of answer detecting using following segments of a question as answers. The results given in Table 5 show that location information is far from adequate to detect contexts and answers. The usefulness of contexts. This experiment is to evaluate the usefulness of contexts in answer detection, by adding the similarity between the context (obtained with different methods) and candidate answer as an extra feature for CRFs. Table 6 shows the impact of context on answer detection using Linear CRFs. Linear CRFs with contextual information perform better than those without context. L-CRF+context is close to that using real context, while it is better than CRFs using the previous sentence as context. The results clearly shows that contextual information greatly improves the performance of answer detection. Improved Models. This experiment is to evaluate the effectiveness of Skip-Chain CRFs (Section 3.2) and 2D CRFs (Section 3.3) for our tasks. The results are given in Table 7 and Table 8. In context detection, Skip-Chain CRFs have simi-

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Model L-CRF+Context Skip-chain 2D 2D+Skip-chain

Prec(%) 75.75 74.18 75.92 76.27

Rec(%) 72.84 74.90 76.54 78.25

F1 (%) 74.45 74.42 76.41 77.34

Model L-CRF+context Skip-chain 2D 2D+Skip-chain

Prec(%) 65.51 67.59 65.77 66.90

Rec(%) 63.13 71.06 68.17 70.56

F1 (%) 64.06 69.40 67.34 68.89

Table 7: Skip-chain and 2D CRFs for context detection

Table 8: Skip-chain and 2D CRFs for answer detection

lar results as Linear CRFs, i.e. the inter-dependency captured by the skip chains generated using the heuristics in Section 3.2 does not improve the context detection. The performance of Linear CRFs is improved in 2D CRFs (by 2%) and 2D+Skip-chain CRFs (by 3%) since they capture the dependency between contiguous questions. In answer detection, as expected, Skip-chain CRFs outperform L-CRF+context since Skip-chain CRFs can model the inter-dependency between contexts and answers while in L-CRF+context the context can only be reflected by the features on the observations. We also observed that 2D CRFs improve the performance of L-CRF+context due to the dependency between contiguous questions. In contrast with our expectation, the 2D+Skip-chain CRFs does not improve Skip-chain CRFs in terms of answer detection. The possible reason could be that the structure of the graph is very complicated and too many parameters need to be learned on our training data. Evaluating Features. We also evaluated the contributions of each category of features in Table 3 to context detection. We found that similarity features are the most important and structural feature the next. We also observed the same trend for answer detection. We omit the details here due to space limitation. As a summary, 1) our CRF model outperforms SVM and C4.5 for both context and answer detections; 2) context is very useful in answer detection; 3) the Skip-chain CRF method is effective in leveraging context for answer detection; and 4) 2D CRF model improves the performance of Linear CRFs for both context and answer detection.

Since contexts of questions are largely unexplored in previous work, we analyze the contexts in our corpus and classify them into three categories: 1) context contains the main content of question while question contains no constraint, e.g. "i will visit NY at
Oct, looking for a cheap hotel but convenient. Any good suggestion? "; 2) contexts explain or clarify part of the question, such as a definite noun phrase, e.g. `We are going on the Taste of Paris. Does anyone know if it is advisable to take a suitcase with us on the tour., where

the first sentence is to describe the tour; and 3) contexts provide constraint or background for question that is syntactically complete, e.g. "We are interested in visiting the Great Wall(and flying from London). Can anyone recommend a tour operator." In our corpus,

5 Discussions and Conclusions
We presented a new approach to detecting contexts and answers for questions in forums with good performance. We next discuss our experience not covered by the experiments, and future work.

about 26% questions do not need context, 12% questions need Type 1 context, 32% need Type 2 context and 30% Type 3. We found that our techniques often do not perform well on Type 3 questions. We observed that factoid questions, one of focuses in the TREC QA community, take less than 10% question in our corpus. It would be interesting to revisit QA techniques to process forum data. Other future work includes: 1) to summarize multiple threads using the triples extracted from individual threads. This could be done by clustering question-context-answer triples; 2) to use the traditional text summarization techniques to summarize the multiple answer segments; 3) to integrate the Question Answering techniques as features of our framework to further improve answer finding; 4) to reformulate questions using its context to generate more user-friendly questions for CQA services; and 5) to evaluate our techniques on more online forums in various domains.

Acknowledgments
We thank the anonymous reviewers for their detailed comments, and Ming Zhou and Young-In Song for their valuable suggestions in preparing the paper.

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