1 Constructing Folksonomies from User- Specified Relations on Flickr Anon Plangprasopchok and Kristina Lerman (WWW 2009)

2 Motivation Users Web content hierarchical classification Consume Produce Annotate Organize Discover Annotation / Metadata Organize Search Recommend Leverage Categorize

3 Motivation Goal: to induce category knowledge from social annotation produced by many users Metadata from an individual user may be too inaccurate and incomplete… The metadata from different users may complement each other, making it, in combination, meaningful.

4 Folksonomy Original definition: classification emerging from the use of tags by users (Thomas Vander Wal) In this work: hidden classification hierarchies from annotation created by many users

5 Hierarchical Relations in Social Web Appear Implicitly Appear Explicitly Tags: Insect Grasshopper Australian Macro Orthopteran ( 直翅類 ) Folder (collection) Sub folder (set) Relations Goal: to induce deeper hierarchies from this metadata

6 Outline Motivation Approaches Results Discussion Related work

7 Inducing Hierarchy from Tags Existing approaches Graph based (Mika05) build a network of associated tags (node = tag, edge = co- occurrence of tags) suggest applying betweenness centrality and set theory to determine broader/narrower relations Hierarchical Clustering (Brooks06; Heymann06+) Tags appear more frequently would have higher centrality and thus more abstract. Probabilistic subsumption ( Sanderson99+, Schmitz06) x is broader than y if x subsumes y x subsumes y if p(x|y) > t & p(y|x) < t x y

8 Inducing Hierarchy from Tags Some difficulties when using tags to induce hierarchy: Above relations induced using subsumption approach on tags [Sanderson99+, Schmitz06] Washington  United States Car  Automobile Notation: A  B (A is broader than B) (hypernym relation) Insect  Hongkong Color  Brazilian Specificity  Rarity Tags are from different facets*

9 Inducing Hierarchy from user-specified relations User specified relations, e.g., –Flickr’s Collection-Set, –Delicious’ Bundle-Tag, –Bibsonomy’s Relation-Tag Key intuition: Not so many people specify peculiar relations like –“automobile”  “car”, or –“Washington”  “United States”

10 Simple Strategy Sets Collection The Netherlands - Holanda Set Collection Blijdorp - Rotterdam Tokenize + Stem … Concept relations netherland holanda blijdorp rotterdam holanda rotterdam countri netherland blijdorp 2. Link concepts & Select path blijdorp countri netherland hollandchina …… 1.Remove “noisy” relations -Conflict resolution -Significance test

11 Remove noisy relations: 1 st approach Conflict Resolution (when both a->b and b->a appear) –Relation conflicts occur because of noise –Voting scheme: Keep a  b (and discard b  a) If N u (a  b) > 1 and N u (a  b) > N u (b  a) insect butterfly insect 10 2

12 Remove noisy relations: 2 nd approach Significance Test - Use statistical significance test to decide if a  b is significant - Null hypothesis: observed relation a  b was generated by chance, via the random, independent generation of individual concepts a, b (according to the binomial distribution). # observations reject accept  # of a  b Is “b” narrower than “a” by chance?

13 Link concepts and select path Link concepts: assume that same terms refer to the same concept. anim bug insect moth possible paths from anim  moth: 1)a  b  i  m 2)a  i  m 3)a  m 4)a  b  m Network Bottleneck idea: “the flow bottleneck is a minimum flow capacity among all relations in the path” 1) a  b  i  m [BN score = min(26,1,18) = 1] 2) a  i  m [BN score = min(72,18) = 18] 3) a  m [BN score = min(10) = 10] 4) a  b  m [BN score = min(26,4) = 4] 10 anim bug + Select path: link relations from many users can cause a spaghetti graph anim insect  anim buginsect

14 Evaluation & Data Set Contribution#2:Learning Concept Hierarchies Hypothesis: the approach that takes explicit relations into account can induce better hierarchies. “Better” means more consistent with hand-built hierarchies (ODP ver. 10/08) The baseline approach is subsumption approach [Schmitz06] Collection and set terms are used instead of tags, making it comparable. Data Set:  Data from 17 user groups, devoted to wildlife and naturalist photography  21,792 of 39,922 users specify at least one collection  110,543 unique terms (c.f. 166,153 unique terms in ODP), 15,495 terms in common.

15 Evaluation methodology ODP has many sub hierarchies: comparing to the induced ones are impractical! Contribution#2:Learning Concept Hierarchies It’s easier to compare when specifying “root concept” and “leaf concepts”, i.e., specifying a certain sub tree to compare. Reference hierarchy Relations (right after tokenized) Induced hierarchy Induce (remove noise+link) (ODP)

16 Metrics Taxonomic Overlap [adapted from Maedche02+] –measuring structure similarity between two trees –for each node, determining how many ancestor and descendant nodes overlap to those in the reference tree. Lexical Recall –measuring how well an approach can discover concepts, existing in the reference hierarchy (coverage) Contribution#2:Learning Concept Hierarchies

17 Quantitative Results

18 Quantitative Results Contribution#2:Learning Concept Hierarchies Manually selecting 32 root nodes Taxonomic Overlap : 27 of them are better than those by subsumption 3 of them get zero score in both approaches Lexical Recall: 28 of them are better than those by subsumption 2 of them get similar score on both approaches the rest, by subsumption, only induce the root node. The proposed approach can induce deeper trees The proposed approach can induce hierarchies more consistent with ODP in almost all cases.

19 Sport hierarchy

20 Invertebrate hierarchy

21 Country hierarchy

22 Discussion Simple strategy to aggregate a large number of shallow relations specified by different users into a common, deeper hierarchy Induced hierarchies are more consistent with ODP Future work includes:  Term ambiguity  Relation types  Global path  Apply to other datasets

23 Thank You! & Questions?