Linear Regression. Robot Image Credit: Viktoriya Sukhanova 123RF.com

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1 Linear Regression These slides were assembled by Eric Eaton, with grateful acknowledgement of the many others who made their course materials freely available online. Feel free to reuse or adapt these slides for your own academic purposes, provided that you include proper attribution. Please send comments and corrections to Eric. Robot Image Credit: Viktoriya Sukhanova 123RF.com

2 Regression Given: n Data X = x (1),...,x (n)o where x (i) 2 R d n Corresponding labels y = y (1),...,y (n)o where y (i) 2 R September Arctic Sea Ice Extent (1,000,000 sq km) Linear Regression Quadratic Regression Year Data from G. Witt. Journal of Statistics Education, Volume 21, Number 1 (2013) 2

3 Linear Regression Hypothesis: y = x x d x d = dx j x j Assume x 0 = 1 j=0 Fit model by minimizing sum of squared errors x Figures are courtesy of Greg Shakhnarovich 3

4 Least Squares Linear Regression Cost Function J( ) = 1 2n Fit by solving min nx i=1 J( ) h x (i) y (i) 2 4

5 Intuition Behind Cost Function J( ) = 1 2n For insight on J(), let s assume nx h x (i) y (i) 2 i=1 so x 2 R =[ 0, 1 ] Based on example by Andrew Ng 5

6 Intuition Behind Cost Function J( ) = 1 2n For insight on J(), let s assume nx h x (i) y (i) 2 i=1 so x 2 R =[ 0, 1 ] (for fixed, this is a function of x) (function of the parameter ) 3 3 y x Based on example by Andrew Ng 6

7 Intuition Behind Cost Function J( ) = 1 2n For insight on J(), let s assume nx h x (i) y (i) 2 i=1 so x 2 R =[ 0, 1 ] (for fixed, this is a function of x) (function of the parameter ) 3 3 y Based on example by Andrew Ng x J([0, 0.5]) = (0.5 1) 2 +(1 2) 2 +(1.5 3)

8 Intuition Behind Cost Function J( ) = 1 2n For insight on J(), let s assume nx h x (i) y (i) 2 i=1 so x 2 R =[ 0, 1 ] y (for fixed, this is a function of x) (function of the parameter ) 3 3 J([0, 0]) J() is concave x Based on example by Andrew Ng 8

9 Intuition Behind Cost Function Slide by Andrew Ng 9

10 Intuition Behind Cost Function (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 10

11 Intuition Behind Cost Function (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 11

12 Intuition Behind Cost Function (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 12

13 Intuition Behind Cost Function (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 13

14 Basic Search Procedure Choose initial value for Until we reach a minimum: Choose a new value for to reduce J( ) J(q 0,q 1 ) q 1 q 0 Figure by Andrew Ng 14

15 Basic Search Procedure Choose initial value for Until we reach a minimum: Choose a new value for to reduce J( ) J(q 0,q 1 ) q 1 q 0 Figure by Andrew Ng 15

16 Basic Search Procedure Choose initial value for Until we reach a minimum: Choose a new value for to reduce J( ) J(q 0,q 1 ) Since the least squares objective function is convex (concave), q 1 q we don t need 0 to worry about local minima Figure by Andrew Ng 16

17 Gradient Descent Initialize Repeat until convergence j J( j simultaneous update for j = 0... d learning rate (small) e.g., α = J( )

18 Gradient Descent Initialize Repeat until convergence j J( j simultaneous update for j = 0... d For j J( ) j 2n nx h x (i) y (i) 2 i=1 X X! 2 18

19 Gradient Descent Initialize Repeat until convergence j J( j simultaneous update for j = 0... d For j J( ) j j 1 2n X nx h x (i) y (i) 2 i=1 nx i=1 X dx k=0 k x (i) k! y (i)! 2 X! 19

20 Gradient Descent Initialize Repeat until convergence j J( j simultaneous update for j = 0... d For j J( ) j j 1 2n = 1 n nx i=1 X nx h x (i) y (i) 2 i=1 nx i=1 dx k=0 X dx k=0 k x (i) k k x (i) k y (i)!! y j dx k=0 k x (i) k y (i)! 20

21 Gradient Descent Initialize Repeat until convergence j J( j simultaneous update for j = 0... d For j J( ) j j 1 2n = 1 n = 1 n nx i=1 nx i=1 nx h x (i) y (i) 2 i=1 nx i=1 dx k=0 dx k=0 dx k=0 k x (i) k k x (i) k k x (i) k y (i) y (i)!! nx y (i) x (i) j dx k=0 k x (i) k y (i)! 21

22 Gradient Descent for Linear Regression Initialize Repeat until convergence j j 1 nx h x (i) n i=1 y (i) x (i) j simultaneous update for j = 0... d To achieve simultaneous update At the start of each GD iteration, compute h x (i) Use this stored value in the update step loop Assume convergence when L 2 norm: s X kvk 2 = vi qv 2 = v v2 v i k new old k 2 < 22

23 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) h(x) = x Slide by Andrew Ng 23

24 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 24

25 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 25

26 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 26

27 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 27

28 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 28

29 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 29

30 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 30

31 Gradient Descent (for fixed, this is a function of x) (function of the parameters ) Slide by Andrew Ng 31

32 Choosing α α too small slow convergence α too large Increasing value for J( ) May overshoot the minimum May fail to converge May even diverge To see if gradient descent is working, print out The value should decrease at each iteration If it doesn t, adjust α J( ) each iteration 32

33 Extending Linear Regression to More Complex Models The inputs X for linear regression can be: Original quantitative inputs Transformation of quantitative inputs e.g. log, exp, square root, square, etc. Polynomial transformation example: y = b 0 + b 1 x + b 2 x 2 + b 3 x 3 Basis expansions Dummy coding of categorical inputs Interactions between variables example: x 3 = x 1 x 2 This allows use of linear regression techniques to fit non- linear datasets.

34 Linear Basis Function Models Generally, h (x) = dx j=0 j j (x) basis function Typically, 0(x) =1so that 0 acts as a bias In the simplest case, we use linear basis functions : j(x) =x j Based on slide by Christopher Bishop (PRML)

35 Linear Basis Function Models Polynomial basis functions: These are global; a small change in x affects all basis functions Gaussian basis functions: These are local; a small change in x only affect nearby basis functions. μ j and s control location and scale (width). Based on slide by Christopher Bishop (PRML)

36 Linear Basis Function Models Sigmoidal basis functions: where These are also local; a small change in x only affects nearby basis functions. μ j and s control location and scale (slope). Based on slide by Christopher Bishop (PRML)

37 Example of Fitting a Polynomial Curve with a Linear Model y = x + 2 x p x p = px j x j j=0

38 Linear Basis Function Models Basic Linear Model: Generalized Linear Model: h (x) = h (x) = Once we have replaced the data by the outputs of the basis functions, fitting the generalized model is exactly the same problem as fitting the basic model Unless we use the kernel trick more on that when we cover support vector machines Therefore, there is no point in cluttering the math with basis functions dx j=0 dx j=0 j x j j j (x) Based on slide by Geoff Hinton 38

39 Linear Algebra Concepts Vector in R d is an ordered set of d real numbers e.g., v = [1,6,3,4] is in R 4 [1,6,3,4] is a column vector: as opposed to a row vector: ( ) æ 1ö ç ç 6 ç 3 ç ç è 4ø An m- by- n matrix is an object with m rows and n columns, where each entry is a real number: æ 1 2 8ö ç ç ç è 9 3 2ø Based on slides by Joseph Bradley

40 Linear Algebra Concepts Transpose: reflect vector/matrix on line: æ aö T ç = è bø ( a b) æ ç è a c b d ö ø T = æ ç è a b c d ö ø Note: (Ax) T =x T A T (We ll define multiplication soon ) Vector norms: L p norm of v = (v 1,,v k ) is Common norms: L 1, L 2 L infinity = max i v i Length of a vector v is L 2 (v) Based on slides by Joseph Bradley X i v i p! 1 p

41 Linear Algebra Concepts Vector dot product: ( u1 u2) ( v1 v2 ) = u1v 1 u2v2 u v = + Note: dot product of u with itself = length(u) 2 = kuk 2 2 Matrix product: A AB æ a = ç ç è a æ a = ç ç è a b b a a ö, ø + + B = a a b b æ b ç ç è b a a b b b b ö ø + + a a b b ö ø Based on slides by Joseph Bradley

42 Vector products: Dot product: Outer product: ( ) v u u v v v u u v u v u T + = ø ö ç ç è æ = = ( ) ø ö ç ç è æ = ø ö ç ç è æ = v u v u u v u v v v u u uv T Based on slides by Joseph Bradley Linear Algebra Concepts

43 Benefits of vectorization Vectorization More compact equations Faster code (using optimized matrix libraries) Consider our model: Let = d h(x) = dx j=0 j x j x = 1 x 1... x d Can write the model in vectorized form as h(x) = x 43

44 Vectorization Consider our model for n instances: h x (i) dx = j x (i) j Let = d R (d+1) X = Can write the model in vectorized form as j=0 1 x (1) 1... x (1) d x (i) 1... x (i) d x (n) 1... x (n) d R n (d+1) h (x) =X 44

45 Vectorization Let: y = For the linear regression cost function: J( ) = 1 nx h x (i) y (i) 2 2n y (1) y (2). y (n) = 1 2n i=1 nx i=1 x (i) y (i) 2 = 1 2n (X y) (X y) R 1 n R n 1 R n (d+1) R (d+1) 1 45

46 Closed Form Solution Instead of using GD, solve for optimal Notice that the solution is when Derivation: Closed Form J( ) analytically ( ) = 1 2n (X y) 1 x 1 J (X y) / X X y X X y + y y / / X X 2 X y + y y Take derivative and set equal to 0, then solve for ( X X 2 X y + y (X X) X y =0 (X X) = X y =(X X) 1 X y 46

47 Closed Form Solution Can obtain by simply plugging X and yinto =(X X) 1 X y X = x (1) 1... x (1) d x (i) 1... x (i) d x (n) 1... x (n) d y = 6 4 If X T X is not invertible (i.e., singular), may need to: Use pseudo- inverse instead of the inverse In python, numpy.linalg.pinv(a) Remove redundant (not linearly independent) features Remove extra features to ensure that d n y (1) y (2). y (n)

48 Gradient Descent vs Closed Form Gradient Descent Requires multiple iterations Need to choose α Works well when n is large Can support incremental learning Closed Form Solution Non- iterative No need for α Slow if n is large Computing (X T X) - 1 is roughly O(n 3 ) 48

49 Improving Learning: Feature Scaling Idea: Ensure that feature have similar scales Before Feature Scaling Makes gradient descent converge much faster After Feature Scaling

50 Feature Standardization Rescales features to have zero mean and unit variance Let μ j be the mean of feature j: µ j = 1 nx x (i) j n Replace each value with: x (i) j x (i) j s j s j is the standard deviation of feature j Could also use the range of feature j (max j min j ) for s j Must apply the same transformation to instances for both training and prediction Outliers can cause problems µ j i=1 for j = 1...d (not x 0!) 50

51 Quality of Fit Price Price Price Size Underfitting (high bias) Size Correct fit Size Overfitting (high variance) Overfitting: The learned hypothesis may fit the training set very well ( J( ) 0 )...but fails to generalize to new examples Based on example by Andrew Ng 51

52 Regularization A method for automatically controlling the complexity of the learned hypothesis Idea: penalize for large values of Can incorporate into the cost function Works well when we have a lot of features, each that contributes a bit to predicting the label Can also address overfitting by eliminating features (either manually or via model selection) j 52

53 Regularization Linear regression objective function J( ) = 1 2n nx i=1 h x (i) y (i) dx j=1 2 j model fit to data regularization is the regularization parameter ( 0) No regularization on 0! 53

54 Understanding Regularization J( ) = 1 2n nx i=1 h x (i) y (i) dx j=1 2 j dx Note that j 2 = k 1:d k 2 2 j=1 This is the magnitude of the feature coefficient vector! We can also think of this as: dx ( j 0) 2 = k 1:d ~0k 2 2 j=1 L 2 regularization pulls coefficients toward 0 54

55 Understanding Regularization J( ) = 1 2n nx i=1 h x (i) y (i) dx j=1 2 j What happens if we set to be huge (e.g., )? Price Size Based on example by Andrew Ng 55

56 Understanding Regularization J( ) = 1 2n nx i=1 h x (i) y (i) dx j=1 2 j What happens if we set to be huge (e.g., )? Price Size Based on example by Andrew Ng 56

57 Regularized Linear Regression Cost Function J( ) = 1 2n Fit by solving nx i=1 min h x (i) y (i) dx J( ) j=1 0 j J( ) Gradient update: n j j 1 n nx i=1 nx i=1 h x (i) y (i) h x (i) y (i) x (i) j j regularization 57

58 Regularized Linear Regression J( ) = 1 2n n nx h x (i) y (i) 2 dx + 2 i=1 j j 1 n nx h x (i) y (i) i=1 nx i=1 h x (i) y (i) x (i) j j=1 2 j j We can rewrite the gradient step as: j j (1 ) 1 nx h x (i) n i=1 y (i) x (i) j 58

59 Regularized Linear Regression To incorporate regularization into the closed form solution: = B X X X y C. 5A

60 Regularized Linear Regression To incorporate regularization into the closed form solution: = B X X X y C. 5A Can derive this the same way, by solving Can prove that for λ > 0, inverse exists in the equation J( ) 60