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- 1. Machine Learning Stephen Scott Associate Professor Dept. of Computer Science University of Nebraska January 21, 2004 Supported by: NSF CCR-0092761 NIH RR-P20 RR17675 NSF EPS-0091900
- 2. What is Machine Learning? Building machines that automatically learn from experience Important research goal of artificial intelligence (Very) small sampling of applications: Data mining programs that learn to detect fraudulent credit card transactions Programs that learn to filter spam email Autonomous vehicles that learn to drive on public highways
- 3. What is Learning? Many different answers, depending on the field you’re considering and whom you ask AI vs. psychology vs. education vs. neurobiology vs. …
- 4. Does Memorization = Learning? Test #1: Thomas learns his mother’s face Memorizes: But will he recognize:
- 5. Thus he can generalize beyond what he’s seen!
- 6. Does Memorization = Learning? (cont’d) Test #2: Nicholas learns about trucks & combines Memorizes: But will he recognize others?
- 7. So learning involves ability to generalize from labeled examples (in contrast, memorization is trivial, especially for a computer)
- 8. Again, what is Machine Learning? Given several labeled examples of a concept E.g. trucks vs. non-trucks Examples are described by features E.g. number-of-wheels (integer), relative-height (height divided by width), hauls-cargo (yes/no) A machine learning algorithm uses these examples to create a hypothesis that will predict the label of new (previously unseen) examples Similar to a very simplified form of human learning Hypotheses can take on many forms
- 9. Hypothesis Type: Decision Tree Very easy to comprehend by humans Compactly represents if-then rules non-truck yes no non-truck non-truck ≥ 4 < 4 ≥ 1 < 1 num-of-wheels hauls-cargo relative-height truck
- 10. Hypothesis Type: Artificial Neural Network Designed to simulate brains “ Neurons” (processing units) communicate via connections, each with a numeric weight Learning comes from adjusting the weights
- 11. Other Hypothesis Types Nearest neighbor Compare new (unlabeled) examples to ones you’ve memorized Support vector machines A new way of looking at artificial neural networks Bagging and boosting Performance enhancers for learning algorithms Many more See your local machine learning instructor for details
- 12. Why Machine Learning? (Relatively) new kind of capability for computers Data mining: extracting new information from medical records, maintenance records, etc. Self-customizing programs: Web browser that learns what you like and seeks it out Applications we can’t program by hand: E.g. speech recognition, autonomous driving
- 13. Why Machine Learning? (cont’d) Understanding human learning and teaching: Mature mathematical models might lend insight The time is right: Recent progress in algorithms and theory Enormous amounts of data and applications Substantial computational power Budding industry (e.g. Google)
- 14. Why Machine Learning? (cont’d) Many old real-world applications of AI were expert systems Essentially a set of if-then rules to emulate a human expert E.g. “If medical test A is positive and test B is negative and if patient is chronically thirsty, then diagnosis = diabetes with confidence 0.85” Rules were extracted via interviews of human experts
- 15. Machine Learning vs. Expert Systems ES: Expertise extraction tedious; ML: Automatic ES: Rules might not incorporate intuition, which might mask true reasons for answer E.g. in medicine, the reasons given for diagnosis x might not be the objectively correct ones, and the expert might be unconsciously picking up on other info ML: More “objective”
- 16. Machine Learning vs. Expert Systems (cont’d) ES: Expertise might not be comprehensive, e.g. physician might not have seen some types of cases ML: Automatic, objective, and data-driven Though it is only as good as the available data
- 17. Relevant Disciplines AI: Learning as a search problem, using prior knowledge to guide learning Probability theory: computing probabilities of hypotheses Computational complexity theory: Bounds on inherent complexity of learning Control theory: Learning to control processes to optimize performance measures Philosophy: Occam’s razor (everything else being equal, simplest explanation is best) Psychology and neurobiology: Practice improves performance, biological justification for artificial neural networks Statistics: Estimating generalization performance
- 18. More Detailed Example: Content-Based Image Retrieval Given database of hundreds of thousands of images How can users easily find what they want? One idea: Users query database by image content E.g. “give me images with a waterfall”
- 19. Content-Based Image Retrieval (cont’d) One approach: Someone annotates each image with text on its content Tedious, terminology ambiguous, maybe subjective Better approach: Query by example Users give examples of images they want Program determines what’s common among them and finds more like them
- 20. Content-Based Image Retrieval (cont’d) User’s Query: System’s Response: Yes Yes Yes NO! User Feedback:
- 21. User’s feedback then labels the new images, which are used as more training examples, yielding a new hypothesis, and more images are retrieved Content-Based Image Retrieval (cont’d)
- 22. How Does the System Work? For each pixel in the image, extract its color + the colors of its neighbors These colors (and their relative positions in the image) are the features the learner uses (replacing e.g. number-of-wheels ) A learning algorithm takes examples of what the user wants, produces a hypothesis of what’s common among them, and uses it to label new images
- 23. Other Applications of ML The Google search engine uses numerous machine learning techniques Spelling corrector: “ spehl korector”, “phonitick spewling”, “Brytney Spears”, “Brithney Spears”, … Grouping together top news stories from numerous sources ( news. google .com ) Analyzing data from over 3 billion web pages to improve search results Analyzing which search results are most often followed, i.e. which results are most relevant
- 24. Other Applications of ML (cont’d) ALVINN, developed at CMU, drives autonomously on highways at 70 mph Sensor input only a single, forward-facing camera
- 25. Other Applications of ML (cont’d) SpamAssassin for filtering spam e-mail Data mining programs for: Analyzing credit card transactions for anomalies Analyzing medical records to automate diagnoses Intrusion detection for computer security Speech recognition, face recognition Biological sequence analysis Each application has its own representation for features, learning algorithm, hypothesis type, etc.
- 26. Conclusions ML started as a field that was mainly for research purposes, with a few niche applications Now applications are very widespread ML is able to automatically find patterns in data that humans cannot However, still very far from emulating human intelligence! Each artificial learner is task-specific
- 27. For More Information Machine Learning by Tom Mitchell, McGraw-Hill, 1997, ISBN: 0070428077 http://www. cse . unl . edu /~sscott See my “hotlist” of machine learning web sites Courses I’ve taught related to ML