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Classification

Classification. Lecture 12. Topics. Classification Frame Terminology and measures Using Classifications In system use In system development Creating Classifications Card sorting. Classification Frame. Classification separates candidates into two or more classes

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Classification

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  1. Classification Lecture 12

  2. Topics • Classification Frame • Terminology and measures • Using Classifications • In system use • In system development • Creating Classifications • Card sorting

  3. Classification Frame • Classification separates candidates into two or more classes • classifying students by grade of degree • We will look at the simple case of two classes first: • filtering Email : Good or Spam • retrieving documents : Relevant or Irrelevant • classifying credit card transactions : Valid or fraudulent • detecting spelling mistakes : ok or mistake (red line) • medical testing : normal or abnormal • Systems Requirement : ambiguous or not abmiguous • METAPHOR : SYSTEM IS A SIEVE

  4. Classification Errors (Information Retrieval) Relevant Irrelevant Retrieved true positive false positive (Type 1 error) false negative (Type II error) true negative Not retrieved Precision = TP/ (TP + FP) = TP/ Retrieved Recall = TP / (TP + FN) = TP / Relevant Efficiency = (TP + TN) / (TP + TN + FP + FN) = (TP+TN) / Full Collection

  5. Example Calculation : email filtering Good Email Spam 7 11 • Precision = TP/ (TP + FP) = • Recall = TP / (TP + FN) = • Efficiency = (TP + TN) / (TP+TN+FP+FN) = accept 3 5 reject

  6. Example Calculation : email filtering Good Email Spam 7 11 • Precision = TP/ (TP + FP) = 3/8 • Recall = TP / (TP + FN) = 3/7 • Efficiency = (TP + TN) / (TP+TN+FP+FN) = 9/18= 50% • Recall > Precision => not quite balanced accept 3 5 FP TP 4 6 reject TN FN

  7. Trade-off • The two errors are usually in conflict • we can decrease the risk of a False Positive (reject more Spam) • but • we increase the risk of False Negatives (rejecting good email) • a TRADE-OFF

  8. Classification Errors Good student Poor student • Write in the terms – relevant, retrieved, true positive, false positive etc Fail Pass

  9. Improved Precision • Precision = TP/ (TP + FP) = TP/ Retrieved • Recall = TP / (TP + FN) = TP / Relevant FN - False Negatives relevant TP -True Positives FP - False Positives TN - True Negatives retrieved

  10. Precision and Recall Full collection • Precision = TP/ (TP + FP) = TP/ Retrieved • Recall = TP / (TP + FN) = TP / Relevant • Efficiency = (TP + TN) / (TP + TN + FP + FN) = (TP+TN) / Full Collection FN - False Negatives relevant TP -True Positives FP - False Positives TN - True Negatives retrieved

  11. Improved Recall • Precision = TP/ (TP + FP) = TP/ Retrieved • Recall = TP / (TP + FN) = TP / Relevant FN - False Negatives relevant TP -True Positives FP - False Positives TN - True Negatives retrieved

  12. Exercise: Precision and Recall in Assessment • Precision means …… • Recall means …. • Ideal values (as %) • Precision= • Recall= • Efficiency • Estimated values • Precision= • Recall= • Efficiency

  13. Classification in the News • Criminal Justice as a Classifer • Murder, Manslaughter or Innocent • What counts as ‘torture’? • Prisoners of war – US invents a new category for the Quantanamo Bay prisoners • Blood groups: • A,B,AB,O • RH+ , RH- • Classification of Cloud types (Cumulus, Cirrus…) by Luke Howard 1802 • Hip evaluation to determine priority for replacement • Programme classification – where does ‘Information Systems’ go?

  14. Categories are Information Structures • Many systems require the user to classify things in the real world into categories in order to process them: • Files and documents into a hierarchical directory structure • Subject matter in a dissertation into sections • Facilities in the University (helpdesk, reception.. • Skills in a Placements system • Budget headings, Nominal Ledger headings • In the computer system, categories can be clearly distinguished: • Codes for each category • In the real world: • categories don’t exist - The fallacy of misplaced concreteness • multiple taxonomies are valid – classifying the same things in different ways for different purposes • Users typically has the task of • mapping the real, complex things into the appropriate categories interpreting categorical information • Implications • Users face a ‘matching’ problem – which category does the item fit best? • IS designers have to devise support for these tasks as well. • Users will not be consistent in their classification (e.g. IS books in Library)

  15. Categories in IS theory • Much of IS theory is based on a taxonomy: • Problem /solution • Method/methodology/technique.. • ER model • Data Flow Diagram • Soft Systems Analysis - CATWOE • Logical /Physical • Swot analysis • Strengths/Weaknesses/Opportunities/Treats • Objective, Goal, Requirement, Constraint

  16. Classification and Systems Design “An early step towards understanding any set of Phenomena is to learn what kinds of things there are in the set – to develop a taxonomy” Herbert Simon • Steps in Classification • defining the domain (what kinds of things are to be classified) • creating the taxonomy (the set of categories), its purpose and force • defining the representation of individuals • defining the mapping between individuals and categories • coding the categories • creating automatic classifiers • assisting human classifiers • assisting users to interpret categorical information • evaluating classification performance • supporting evolution of taxonomy and classifiers

  17. A Poor Classification? • The Argentinean writer Jorge Luis Borges ‘Imaginary Beasts’, ‘Labyrinths’..) quotes a ‘certain Chinese encyclopaedia’ in which animals are divided into: A) belonging to the Emperor B) embalmed C) tame D) suckling pigs E) sirens F) fabulous G) stray dogs H) included in the present classification I) frenzied J) innumerable K) drawn with a very fine camel hair brush L) et cetera M) having just broken the water pitcher N) that from a long way off look like flies

  18. Machine Classifier Human Categories/Classes A B C Taxonomy

  19. Categories not Mutually Exclusive Machine An object can be put in any of several categories Classifier Human Categories/Classes A B C Taxonomy

  20. Categories not Complete Machine Classifier Some objects don’t belong anywhere Human Categories/Classes A B C Taxonomy

  21. Categories not Balanced Machine Some categories much larger than others Classifier Human Categories/Classes A B C Taxonomy

  22. Categories Inconsistant Machine Categories lack a single organising principle Classifier Human Categories/Classes A B C Taxonomy

  23. Characteristics of a good Taxonomy • Categories must be: • Mutually exclusive • Every object in at most one category • Complete (exhaustive) • Every object in at least one category • Balanced • Categories divide objects evenly • Consistent • Same characteristics used throughout • Hierarchical integrity • Categories at one level not confused with categories at another level

  24. Kinds of classification • Classical • Classes defined by presence of features • Square : 4 sides, equal length, equal angles • Triangle : 3 sides, equal length, equal angles • Probabilistic • Classes defined by weighted sum of features • ‘bird’ moves, winged, feathered, sings, lays eggs • Is a robin a bird? Is a emu a bird? • Exemplar (prototype) • Classes defined by one or more key examples • Robin is a central example of ‘bird’ • Chicken is more remote example • Which kind is used in IS Theory? • Which kind is used in IS Use?

  25. Automated Clustering • Clustering techniques find groups of similar objects • Used in data mining to identify customer groups with similar buying behaviour… • Mathematical Techniques • k-nearest neighbour • ID3 to create decision tree • Human Techniques • Card sorting

  26. Classifying • Learning Classifiers • Based on sample of population • Classified by hand • Split into two parts • The training set used to compute the classifier • The test set used to test the ability of the classifier • Many kinds of classifiers available, all need good understanding of statistics e.g. Naïve Bayesian, Decision Tree, SVM • Threshold set to balance recall and precision • Rule and example based for human classifier but performance varies with experience and skill • E.g. book classification, Yahoo directory classification, medical diagnosis • Human classifiers need to be trained too • If classification done by end-users, classification is likely to be inconsistent

  27. Review • 3 tier web architecture – describe, explain, terminology, typical interactions • SQL & PHP • Extended ER models • Interaction in human and computer systems – sequence diagrams, state-full interaction • Alternative Development Processes –Agile Development and Extreme Programming – description, application, comparison with SSADM, choice of appropriate development model • Frames – rationale, role in IS development, basic recognition in a problem description of simple frames and the following in detail • Matching Frame – typical applications, fitness function, recognising nominal, ordinal, interval and ratio scales, use of weights • Classification Frame – typical applications, terminology, calculation of recall and precision, guidelines for constructing a taxonomy

  28. Preview • XML and XSLT • Business Processes and BPML • Scenarios and Use cases • Learning Frame

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