We are delighted to announce that our poster “ParsRec: Meta-Learning Recommendations for Bibliographic Reference Parsing” has been accepted at the 12th ACM Recommender Systems Conference (RecSys) for presentation in Vancouver, Canada. The pre-print is available on arXiv, and here in our blog:
Bibliographic reference parsers extract metadata (e.g. author names, title, year) from bibliographic reference strings. No reference parser consistently gives the best results in every scenario. For instance, one tool may be best in extracting titles, and another tool in extracting author names. In this paper, we address the problem of reference parsing from a recommender-systems perspective. We propose ParsRec, a meta-learning approach that recommends the potentially best parser(s) for a given reference string. We evaluate ParsRec on 105k references from chemistry. We propose two approaches to meta-learning recommendations. The first approach learns the best parser for an entire reference string. The second approach learns the best parser for each field of a reference string. The second approach achieved a 2.6% increase in F1 (0.909 vs. 0.886, p < 0.001) over the best single parser (GROBID), reducing the false positive rate by 20.2% (0.075 vs. 0.094), and the false negative rate by 18.9% (0.107 vs. 0.132).
Bibliographic reference parsing is a well-known task in scientific information extraction. In reference parsing, the input is a single reference string, formatted in a specific bibliography style (Figure 1). The output is a machine-readable representation of the input string, typically called a parsed reference. A parsed reference is a collection of metadata fields, each of which is composed of a metadata type (e.g. “year” or “conference”) and value (e.g. “2018” or “RecSys”). Reference parsing is important for academic search engines and recommender systems.
Figure 1: An example bibliographic reference string on the input of reference parsing.
There exist many reference parser tools, and their quality varies greatly, depending on the metadata field and other factors. For example, in our previous study , ParsCit was best in extracting authors but only third best over all fields, and Science Parse was best in extracting the year but only fourth best over all fields. Consequently, if we were able to choose the best parser for a given scenario, the overall quality of the results should increase. This can be seen as a typical recommendation problem: a user (e.g. a software developer) needs the item (reference parser) that satisfies the user‘s needs best (high quality parsing results).
Meta-learning is often applied to the problem of algorithm selection . Meta-learning allows the training of a model able to select the best algorithm for a given problem. As far as we know, meta-learning has not been applied to reference parsing.
We introduce ParsRec, a novel meta-learning approach for recommending bibliographic reference parsers. ParsRec takes as input a reference string, identifies the potentially best parser(s) for this string, applies the chosen parser(s), and outputs the extracted metadata fields. ParsRec is built upon 10 open-source parsers: Anystyle-Parser, Biblio, CERMINE, Citation, Citation-Parser, GROBID, ParsCit, PDFSSA4MET, Reference Tagger and Science Parse. ParsRec uses supervised machine learning to recommend the best parser(s) to a reference string. From a recommender-systems perspective, ParsRec can be seen as a switching hybrid ensemble  of reference parsers, where the switching is controlled by machine learning. The novel aspects of ParsRec are: 1) considering reference parsing as a recommendation problem, 2) using a meta learning approach for reference parsing.
We propose and evaluate two meta-learning reference recommendation approaches, being inspired by .
ParsRecRef recommends the potentially best parser for an entire reference string in 3 steps (Figure 2). First, for each parser it uses a linear regression model to predict the performance of the parser (measured by F1) on the given reference string. Second, it ranks the parsers by predicted performance. Finally, it chooses the parser ranked most highly, and applies it to the input string.
ParsRecRef uses two types of features to represent reference strings: 9 heuristics and 150 n-grams. The heuristics are: reference string length, number and fraction of commas, dots, and semicolons, and whether the string starts with a square bracket (e.g. “”), or a dot enumeration pattern (e.g. “14.”). N-gram features are 3- and 4-grams, where the terms are classes of words such as number, capword (capitalized word), comma, etc. These features capture style-characteristic sequences, such as number-comma-number (matching “3, 12”), capword-comma-upperlett-dot (matching “Spring, B.”). The n-gram features are automatically chosen based on random forest’s feature importance.
Figure 2: The workflow of ParsRecRef.
ParsRecField recommends a reference parser for each metadata type in the input reference string (Figure 3). First, ParsRecField iterates over all pairs (parser, metadata type), and for each pair it predicts whether the parser will correctly extract the metadata type from the input reference string. The prediction of correctness is done by a logistic regression model, trained separately for each pair (parser, metadata type). Second, for each metadata type, ParsRecField ranks the parsers based on predicted probability of correct extraction, and chooses the parser ranked most highly. All chosen parsers are applied to the input string and the fields are chosen according to previous choice of the parsers.
Figure 3: The workflow of ParsRecField.
Evaluation and Results
The dataset used for the experiments comes from a business project and was manually curated. The dataset is composed of 371,656 references from chemical domains (strings and parsed versions) and 1.9 million metadata fields. The dataset contains 6 metadata types: author, source, year, volume, issue, and page.
The data was divided as follows: 40% for the training of individual parsers (out of scope of this paper), 30% for the training of the recommender (meta-learning), and 30% for testing.
We compare ParsRec against three baselines. The first baseline is the best single parser (GROBID). The second baseline, a hybrid baseline, uses the best parser for each metadata type separately, according to . The third baseline is a voting ensemble, in which the final result contains metadata fields extracted by at least 3 parsers. We report the results in terms of precision, recall and F1, calculated over the metadata fields.
The overall results are presented in Figure 4. Both variations of ParsRec outperform the best single parser. ParsRecRef achieved a 0.6% increase in F1 (0.891 vs. 0.886), reducing the false positive rate by 3.2% (0.091 vs. 0.094), and the false negative rate by 3.8% (0.127 vs. 0.132). ParsRecField achieved a 2.6% increase in F1 (0.909 vs. 0.886), reducing the false positive rate by 20.2% (0.075 vs. 0.094), and the false negative rate by 18.9% (0.107 vs. 0.132). Both increases in F1 are statistically significant (t-test, p = 0.0027 for ParsRecRef and p < 0.001 for ParsRecField).
Figure 4: The results of ParsRec and the three baselines.
Both versions outperform the voting ensemble. While ParsRecRef is better by only 0.1 percentage points (F1 0.890 vs. 0.891, not significant), ParsRecField achieved a 2.1% increase in F1 (0.909 vs. 0.890, p < 0.001). ParsRecField also outperforms the hybrid baseline witha 1.6% increase in F1 (0.909 vs. 0.895, p < 0.001).
Our evaluation demonstrates the potential of meta learning and the application of recommendation techniques to reference parsing. Our two approaches outperform the best single parser and voting ensemble, and ParsRecField outperforms all baselines. These results indicate that ParsRec makes good recommendations. In most cases, the increases in F1 are statistically significant, though not high. We suspect the reason for this is low diversity in the data (only chemical papers) and among the parsers (six out of 10 parsers use Conditional Random Fields).
 D. Tkaczyk, A. Collins, P. Sheridan and J. Beel, “Machine Learning vs. Rules and Out-of-the-Box vs. Retrained: An Evaluation of Open-Source Bibliographic Reference and Citation Parsers,” in Joint Conference on Digital Libraries, 2018.
 C. Lemke, M. Budka and B. Gabrys, “Metalearning: a survey of trends and technologies,” Artificial Intelligence Review, vol. 44, no. 1, pp. 117-130, 2015.
 R. D. Burke, “Hybrid Web Recommender Systems,” in The Adaptive Web, Methods and Strategies of Web Personalization, 2007.
 A. Collins, J. Beel and D. Tkaczyk, “One-at-a-time: A Meta-Learning Recommender-System for Recommendation-Algorithm Selection on Micro Level,” CoRR, vol. abs/1805.12118.