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- 1. INTRODUCTION TO DATA SCIENCE NIKO VUOKKO JYVÄSKYLÄ SUMMER SCHOOL AUGUST 2013
- 2. DATA SCIENCE WITH A BROAD BRUSH Concepts and methodologies
- 3. DATA SCIENCE IS AN UMBRELLA, A FUSION • Databases and infrastructure • Pattern mining • Statistics • Machine learning • Numerical optimization • Stochastic modeling • Data visualization … of specialties needed for data-driven business optimization
- 4. DATA SCIENTIST • Data scientist is defined as DS : business problem data solution • Combination of strong programming, math, computational and business skills • Recipe for success 1. Convert vague business requirements into measurable technical targets 2. Develop a solution to reach the targets 3. Communicate business results 4. Deploy the solution in production
- 5. UNDERSTANDING DATA Monday 19 August 2013
- 6. PATTERN MINING AND DATA ANALYSIS
- 7. UNSUPERVISED LEARNING • Could be called pattern recognition or structure discovery • What kind of a process could have produced this data? • Discovery of “interesting” phenomena in a dataset • Now how do you define interesting? • Learning algorithms exist for a huge collection of pattern types • Analogy: You decide if you want to see westerns or comedies, but the machine picks the movies • But does “interesting” imply useful and significant?
- 8. EXAMPLES OF STRUCTURES IN DATA • Clustering and mixture models: separation of data into parts • Dictionary learning: a compact grammar of the dataset • Single class learning: learn the natural boundaries of data Example: Early detection of machine failure or network intrusion • Latent allocation: learn hidden preferences driving purchase decisions • Source separation: find independent generators of the data Example: Independent phenomena affecting exchange rates
- 9. MORE EXAMPLES OF “INTERESTING” PATTERNS • { charcoal, mustard } ⇒ sausage • Grocery customer types with differing paths around the trading floor • Pricing trend change in a web ad exchange • Communities and topics in a social network • Distinct features of a person’s face and fingerprints • Objects emerging in front of a moving car
- 10. KNOW YOUR EIGENS AND SINGULARS • Eigenvalue and singular value decompositions are central data analysis tools • They describe the energy distribution and static core structures of data Examples • Face detection, speaker adaptation • Google PageRank is basically just the world’s largest EVD • Zombie outbreak risk is determined by its eigenvalues • As a sub-component in every second learning algorithm
- 11. DIMENSION REDUCTION • Some applications encounter large dimension counts up to millions • Dimension reduction may either 1. Retain space: preserve the most “descriptive” dimensions 2. Transform space: trade interpretability for powerful rendition • Usually transformations work oblivious to data (they are simple functions) • Curvilinear transformations try to see how the data is “folded” and build new dimensions specific to the given dataset
- 12. DIMENSION REDUCTION EXAMPLE • Singular value decomposition is commonly used to remove the “noise dimensions” with little energy • Example: gene expression data and movie preferences have lots of these • After this more complex methods can be used for unfolding the data
- 13. DIMENSION REDUCTION EXAMPLE
- 14. BLIND SOURCE SEPARATION • Find latent sources that generated the data • Tries to discover the real truth beneath all noise and convolution • Examples: • Air defense missile guidance systems • Error-correcting codes • Language modeling • Brain activity factors • Industrial process dynamics • Factors behind climate change
- 15. (STATISTICAL) SIGNIFICANCE TESTING • Example: Rejection rate increase in a manufacturing plant • “What is the probability of observing this increase if everything was OK?” • “What is the probability of having a valid alert if there really was something wrong?” • Reliability of significance testing results is wholly dependent on correct modeling of the data source and pattern type • Statistical significance is different from material significance
- 16. CORRELATION IS NOT CAUSALITY A correlation may hide an almost arbitrary truth • Cities with more firemen have more fires • Companies spending more in marketing have higher revenues • Marsupials exist mainly in Australia • However, making successful predictions does not require causality
- 17. MACHINE LEARNING Basics
- 18. SUPERVISED LEARNING • Simplistically task is to find function f : f(input) = output • Examples: spam filtering, speech recognition, steel strength estimation • Risks for different types of errors can be very skewed • Complex inputs may confuse or slow down models • Unsupervised methods often useful in improving results by simplifying the input
- 19. SEMI-SUPERVISED LEARNING • Only a part of data is labeled • Needed when labeling data is expensive • Understanding the structure of unlabeled data enhances learning by bringing diversity and generalization and by constraining learning • Relates to multi-source learning, some sources labeled, some not • Examples: • Object detection from a video feed • Web page categorization • Sentiment analysis • Transfer learning between domains
- 20. TRAINING, TESTING, VALIDATION • A model is trained using a training dataset • The quality of the model is measured by using it on a separate testing dataset • A model often contains hyper-parameters chosen by the user • A separate validation dataset is split off from the training data • Validation data is used for testing and finding good hyper-parameter values • Cross-validation is common practice and asymptotically unbiased
- 21. BIAS AND VARIANCE • Squared error of predictions consists of bias and variance (and noise) • BIAS Model incapability of approximating the underlying truth • VARIANCE Model reliance on whims of the observed data • Complex models often have low bias and high variance • Simple models often have high bias and low variance • Having more data instances (rows) may reduce variance • Having more detailed data (variables) may reduce bias • Testing different types of models can explain how to improve your data
- 22. TRAINING AND TESTING, BIAS AND VARIANCE Complex modelSimple model Minimal testing error Minimal training error
- 23. MACHINE LEARNING Learning new tricks
- 24. THE KERNEL TRICK • Many learning methods rely on inner products of data points • The “kernel trick” maps the data to an implicitly defined, high dimension space • Kernel is the matrix of the new inner products in this space • Mapping itself often left unknown • Example: Gaussian kernel associates local Euclidean neighborhoods to similarity • Example: String kernels are used for modeling DNA sequence structure • Kernels can be combined and custom built to match expert knowledge A kernel is a dataset-specific space transformation, success depends on good understanding of the dataset
- 25. ENSEMBLE LEARNING • The power of many: combine multiple models into one • Wide and strong proof of superior performance • Extra bonus: often trivially parallelizable OUR EXPERIENCE IS THAT MOST EFFORTS SHOULD BE CONCENTRATED IN DERIVING SUBSTANTIALLY DIFFERENT APPROACHES, RATHER THAN REFINING A SINGLE TECHNIQUE. Netflix $1M prize winner (ensemble of 107 models) “ “
- 26. ENSEMBLE LEARNING IN PRACTICE • Boosting: weigh (⇒ low bias) focused (⇒ low bias) simple models (⇒ low bias) • Bagging: average (⇒ low variance) results of simple models (⇒ low bias) • What aspect of the data am I still missing? • Variable mixing, discretized jumps, independent factors, transformations, etc. • Questions about practical implementability and ROI • Failure: Netflix winner solution never taken to production • Success: Official US hurricane model is an ensemble of 43
- 27. RANDOMIZED LEARNING • Motivation: random variation beats expert guidance surprisingly often • Introducing randomness can improve generalization performance (smaller variance) • Randomness allows methods to discover unexpected success • Examples: genetic models, simulated annealing, parallel tempering • Increasingly useful to allow scale-out for large datasets • Many successful methods combine random models as an ensemble • Example: combining random projections or transformations can often beat optimized unsupervised models
- 28. ONLINE LEARNING • Instead of ingesting a training dataset, adjust the data model after every incoming (instance, label) pair • Allows quick adaptation and “always-on” operation • Finds good models fast, but may miss the great one ⟹ suitable also as a burn-in for other models • Useful especially for the present trend towards analyzing data streams
- 29. BAYESIAN BASICS • Bayesians see data as fixed and parameters as distributions • Parameters have prior assumptions that can encode expert knowledge • Data is used as evidence for possible parameter values • Final output is a set of posterior distributions for the parameters • Models may employ only the most probable parameter values or their full probability distribution • Variational Bayes approximates the posterior with a simpler distribution
- 30. MODEL COMPLEXITY • Limiting model size and complexity can be used to avoid excessive bias • Minimum description length and Akaike/Bayesian information criteria are the Occam’s razor of data science • VC dimension of a model provides a theoretical limit for generalization error • Regularization can limit instance weights or parameter sizes • Bayesian models use hyper-parameters to limit parameter overfit
- 31. THE END