# An Introduction to Matrix Algebra

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1 An Introduction to Matrix Algebra EPSY 905: Fundamentals of Multivariate Modeling Online Lecture #8 EPSY 905: Matrix Algebra

2 In This Lecture An introduction to matrix algebra Ø Scalars, vectors, and matrices Ø Basic matrix operations Ø Advanced matrix operations An introduction to matrices in R Ø Embedded within the R language EPSY 905: Matrix Algebra 2

3 Why Learning a Little Matrix Algebra is Important Matrix algebra is the alphabet of the language of statistics Ø You will most likely encounter formulae with matrices very quickly For example, imagine you were interested in analyzing some repeated measures data but things don t go as planned Ø From the SAS User s Guide (PROC MIXED): EPSY 905: Matrix Algebra 3

4 Introduction and Motivation Nearly all multivariate statistical techniques are described with matrix algebra When new methods are developed, the first published work typically involves matrices Ø It makes technical writing more concise formulae are smaller Have you seen: Ø! "! #\$! " % Ø &'& ( + * Useful tip: matrix algebra is a great way to get out of boring conversations and other awkward moments EPSY 905: Matrix Algebra 4

5 Definitions We begin this class with some general definitions (from dictionary.com): Ø Matrix: 1. A rectangular array of numeric or algebraic quantities subject to mathematical operations 2. The substrate on or within which a fungus grows Ø Algebra: 1. A branch of mathematics in which symbols, usually letters of the alphabet, represent numbers or members of a specified set and are used to represent quantities and to express general relationships that hold for all members of the set 2. A set together with a pair of binary operations defined on the set. Usually, the set and the operations include an identity element, and the operations are commutative or associative EPSY 905: Matrix Algebra 5

6 Why Learn Matrix Algebra Matrix algebra can seem very abstract from the purposes of this class (and statistics in general) Learning matrix algebra is important for: Ø Understanding how statistical methods work w And when to use them (or not use them) Ø Understanding what statistical methods mean Ø Reading and writing results from new statistical methods This is a first lecture of learning the language of multivariate statistics EPSY 905: Matrix Algebra 6

7 DATA EXAMPLE AND R EPSY 905: Matrix Algebra 7

8 A Guiding Example To demonstrate matrix algebra, we will make use of data Imagine that I collected data SAT test scores for both the Math (SATM) and Verbal (SATV) sections of 1,000 students The descriptive statistics of this data set are given below: EPSY 905: Matrix Algebra 8

9 The Data In Excel: In R: EPSY 905: Matrix Algebra 9

10 DEFINITIONS OF MATRICES, VECTORS, AND SCALARS EPSY 905: Matrix Algebra 10

11 Matrices A matrix is a rectangular array of data Ø Used for storing numbers Matrices can have unlimited dimensions Ø For our purposes all matrices will have two dimensions: w Row w Columns Matrices are symbolized by boldface font in text, typically with capital letters Ø Size (r rows x c columns) SAT Verbal (Column 1) SAT Math (Column 2)! = (+,,, -.) EPSY 905: Matrix Algebra 11

12 Vectors A vector is a matrix where one dimension is equal to size 1 Ø Column vector: a matrix of size! " \$ %& = 540 &---. & Ø Row vector: a matrix of size 1 " / \$ &% = &. 1 Vectors are typically written in boldface font text, usually with lowercase letters The dots in the subscripts \$ %& and \$ &% represent the dimension aggregated across in the vector Ø \$ &% is the first row and all columns of 2 Ø \$ %& is the first column and all rows of 2 Ø Sometimes the rows and columns are separated by a comma (making it possible to read double-digits in either dimension) EPSY 905: Matrix Algebra 12

13 Matrix Elements A matrix (or vector) is composed of a set of elements Ø Each element is denoted by its position in the matrix (row and column) For our matrix of data! (size 1000 rows and 2 columns), each element is denoted by: " #\$ Ø The first subscript is the index for the rows: i = 1,,r (= 1000) Ø The second subscript is the index for the columns: j = 1,,c (= 2)! = " && " &' " '& " '' " &))),& " &))),' (&))), ') EPSY 905: Matrix Algebra 13

14 Scalars A scalar is just a single number The name scalar is important: the number scales a vector it can make a vector longer or shorter Scalars are typically written without boldface:! "" = 520 Each element of a matrix is a scalar EPSY 905: Matrix Algebra 14

15 Matrix Transpose The transpose of a matrix is a reorganization of the matrix by switching the indices for the rows and columns ! = (+,,, -.)! 0 = ,,, An element 2 34 in the original matrix! is now 2 43 in the transposed matrix! 0 Transposes are used to align matrices for operations where the sizes of matrices matter (such as matrix multiplication) EPSY 905: Matrix Algebra 15

16 Types of Matrices Square Matrix: A square matrix has the same number of rows and columns Ø Correlation/covariance matrices are square matrices Diagonal Matrix: A diagonal matrix is a square matrix with non-zero diagonal elements (! "# 0 for ) = +) and zeros on the off-diagonal elements (! "# = 0 for ) +): = Ø We will use diagonal matrices to form correlation matrices Symmetric Matrix: A symmetric matrix is a square matrix where all elements are reflected across the diagonal (8 "# = 8 #" ) Ø Correlation and covariance matrices are symmetric matrices EPSY 905: Matrix Algebra 16

17 VECTORS EPSY 905: Matrix Algebra 17

18 Vectors in Space Vectors (row or column) can be represented as lines on a Cartesian coordinate system (a graph) Consider the vectors:! = 1 2 and % = 2 3 A graph of these vectors would be: Question: how would a column vector for each of our example variables (SATM and SATV) be plotted? EPSY 905: Matrix Algebra 18

19 Vector Length The length of a vector emanating from the origin is given by the Pythagorean formula Ø This is also called the Euclidean distance between the endpoint of the vector and the origin! " = \$ & %% + \$ & &% + + \$ & )% = " From the last slide: * = 5 = 2.24; / = 13 = 3.61 From our data: 3456 = 15, ; 345: = 15, In data: length is an analog to the standard deviation Ø In mean-centered variables, the length is the square root of the sum of mean deviations (not quite the SD, but close) EPSY 905: Matrix Algebra 19

20 Vector Addition Vectors can be added together so that a new vector is formed Vector addition is done element-wise, by adding each of the respective elements together: Ø The new vector has the same number of rows and columns! = # + % = = 3 5 Ø Geometrically, this creates a new vector along either of the previous two w Starting at the origin and ending at a new point in space In Data: a new variable (say, SAT total) is the result of vector addition *+,,-,+. = /.1 + /.2 EPSY 905: Matrix Algebra 20

21 Vector Addition: Geometrically EPSY 905: Matrix Algebra 21

22 Vector Multiplication by Scalar Vectors can be multiplied by scalars Ø All elements are multiplied by the scalar! = 2\$ = = 2 4 Scalar multiplication changes the length of the vector:! = 2 ' + 4 ' = 20 = 4.47 This is where the term scalar comes from: a scalar ends up rescaling (resizing) a vector In Data: the GLM (where X is a matrix of data) the fixed effects (slopes) are scalars multiplying the data EPSY 905: Matrix Algebra 22

23 Scalar Multiplication: Geometrically EPSY 905: Matrix Algebra 23

24 Linear Combinations Addition of a set of vectors (all multiplied by scalars) is called a linear combination:! = # \$ % \$ + # ' % ' + + # ) % ) Here,! is the linear combination Ø For all k vectors, the set of all possible linear combinations is called their span Ø Typically not thought of in most analyses but when working with things that don t exist (latent variables) becomes somewhat important In Data: linear combinations happen frequently: Ø Linear models (i.e., Regression and ANOVA) Ø Principal components analysis EPSY 905: Matrix Algebra 24

25 Linear Dependencies A set of vectors are said to be linearly dependent if! " # " +! % # % + +! ' # ' = 0 -and-! ",! %,,! ' are all not zero Example: let s make a new variable SAT Total:,-. /0/12 = 1, ,-.6 The new variable is linearly dependent with the others: 1, ,-.6 + ( 1),-. /0/12 = : In Data: (multi)collinearity is a linear dependency. Linear dependencies are bad for statistical analyses that use matrix inverses EPSY 905: Matrix Algebra 25

26 Inner (Dot) Product of Vectors An important concept in vector geometry is that of the inner product of two vectors Ø The inner product is also called the dot product! " # =! % # = & '' ( '' + & *' ( *' + + &,' (,' = - &.' (.',./' The dot or inner product is related to the angle between vectors and to the projection of one vector onto another From our example:! " # = = 8 From our data: 5.' 5.* = 251,928,400 In data: the angle between vectors is related to the correlation between variables and the projection is related to regression/anova/linear models EPSY 905: Matrix Algebra 26

27 Angle Between Vectors As vectors are conceptualized geometrically, the angle between two vectors can be calculated! "# = cos () * +, * - From the example:! "# = cos () = 0.12 From our data:! 5678,567: = cos () 251,928,400 15, , = EPSY 905: Matrix Algebra 27

28 In Data: Cosine Angle = Correlation If you have data that are: Ø Placed into vectors Ø Centered by the mean (subtract the mean from each observation) then the cosine of the angle between those vectors is the correlation between the variables:! "# = %&' ( "# = ) * + ), = 1./ / / For the SAT example data (using mean centered variables):! 789:,789< = cos ( 3,132,223.6 = cos 1, , =.775 EPSY 905: Matrix Algebra 28

29 Vector Projections A final vector property that shows up in statistical terms frequently is that of a projection The projection of a vector! onto " is the orthogonal projection of! onto the straight line defined by " Ø The projection is the shadow of one vector onto the other:! #\$%& " =! ( " " ) " In data: linear models can be thought of as projections EPSY 905: Matrix Algebra 29

30 Vector Projections Example To provide a bit more context for vector projections, let s consider the projection of mean centered SATV onto SATM:!"#\$%,!"#*%!"#\$% &'()!"#*% =!"#*% -!"#*% The first portion turns out to be:!"#\$%,!"#*%!"#*% - = 3,132, , =.475 This is also the regression slope 8 9 : :;<= > = 8? :;<A > + B > EPSY 905: Matrix Algebra 30

31 MATRIX ALGEBRA EPSY 905: Matrix Algebra 31

32 Moving from Vectors to Matrices A matrix can be thought of as a collection of vectors Ø Matrix operations are vector operations on steroids Matrix algebra defines a set of operations and entities on matrices Ø I will present a version meant to mirror your previous algebra experiences Definitions: Ø Identity matrix Ø Zero vector Ø Ones vector Basic Operations: Ø Addition Ø Subtraction Ø Multiplication Ø Division EPSY 905: Matrix Algebra 32

33 Matrix Addition and Subtraction Matrix addition and subtraction are much like vector addition/subtraction Rules: Ø Matrices must be the same size (rows and columns) Method: Ø The new matrix is constructed of element-by-element addition/subtraction of the previous matrices Order: Ø The order of the matrices (pre- and post-) does not matter EPSY 905: Matrix Algebra 33

34 Matrix Addition/Subtraction EPSY 905: Matrix Algebra 34

35 Matrix Multiplication Matrix multiplication is a bit more complicated Ø The new matrix may be a different size from either of the two multiplying matrices! (# \$ %) ' (% \$ () = * (# \$ () Rules: Ø Pre-multiplying matrix must have number of columns equal to the number of rows of the post-multiplying matrix Method: Ø The elements of the new matrix consist of the inner (dot) product of the row vectors of the pre-multiplying matrix and the column vectors of the postmultiplying matrix Order: Ø The order of the matrices (pre- and post-) matters EPSY 905: Matrix Algebra 35

36 Matrix Multiplication EPSY 905: Matrix Algebra 36

37 Multiplication in Statistics Many statistical formulae with summation can be re-expressed with matrices A common matrix multiplication form is:! "! ' ( \$ ) Ø Diagonal elements: \$%& Ø Off-diagonal elements: ' \$%& ( \$* ( \$+ For our SAT example:! "! = = ' ' ) -./01 \$ -./01 \$./02 \$ \$%& ' \$%& ) -./01 \$./02 \$ -./02 \$ \$%& ' \$%& 251,797, ,928, ,928, ,862,700 EPSY 905: Matrix Algebra 37

38 Identity Matrix The identity matrix is a matrix that, when pre- or postmultiplied by another matrix results in the original matrix:!" =! "! =! The identity matrix is a square matrix that has: Ø Diagonal elements = 1 Ø Off-diagonal elements = \$ % & % = EPSY 905: Matrix Algebra 38

39 Zero Vector The zero vector is a column vector of zeros! (# \$ %) = When pre- or post- multiplied the result is the zero vector: )! =!!) =! EPSY 905: Matrix Algebra 39

40 Ones Vector A ones vector is a column vector of 1s:! (# \$ %) = The ones vector is useful for calculating statistical terms, such as the mean vector and the covariance matrix EPSY 905: Matrix Algebra 40

41 Matrix Division : The Inverse Matrix Division from algebra: Ø First:! " = \$ " % = &'\$ % Ø Second:!! = 1 Division in matrices serves a similar role Ø For square and symmetric matrices, an inverse matrix is a matrix that when pre- or post- multiplied with another matrix produces the identity matrix: ) '\$ ) = * )) '+ = * Calculation of the matrix inverse is complicated Ø Even computers have a tough time Not all matrices can be inverted Ø Non-invertible matrices are called singular matrices w In statistics, singular matrices are commonly caused by linear dependencies EPSY 905: Matrix Algebra 41

42 The Inverse In data: the inverse shows up constantly in statistics Ø Models which assume some type of (multivariate) normality need an inverse covariance matrix Using our SAT example Ø Our data matrix was size (1000 x 2), which is not invertible Ø However! "! was size (2 x 2) square, and symmetric! " 251,797, ,928,400! = 251,928, ,862,700 Ø The inverse is:! "!./ = EPSY 905: Matrix Algebra 42

43 Matrix Algebra Operations! + # + \$ =! + (# + \$)! + # = # +! (! + # = (! + (# ( + )! = (! + )!! + # * =! * + # * ()! = (()!) (! * = (! * (!# = (! #! #\$ =!# \$! # + \$ =!# +!\$!# * = # *! * For +, such that -+, exists: 1.!2, =!.,/0 1.,/0 1,/0 2,!2,!2, * = 1 *!. 2, 2,,/0! * EPSY 905: Matrix Algebra 43

44 ADVANCED MATRIX OPERATIONS EPSY 905: Matrix Algebra 44

45 Advanced Matrix Functions/Operations We end our matrix discussion with some advanced topics Ø All related to multivariate statistical analysis To help us throughout, let s consider the correlation matrix of our SAT data: ! = EPSY 905: Matrix Algebra 45

46 Matrix Trace For a square matrix! with p rows/columns, the trace is the sum of the diagonal elements: ) "#! = % &'( For our data, the trace of the correlation matrix is 2 Ø For all correlation matrices, the trace is equal to the number of variables because all diagonal elements are 1 * && The trace is considered the total variance in multivariate statistics Ø Used as a target to recover when applying statistical models EPSY 905: Matrix Algebra 46

47 Matrix Determinants A square matrix can be characterized by a scalar value called a determinant: det \$ = \$ Calculation of the determinant is tedious Ø Our determinant was The determinant is useful in statistics: Ø Shows up in multivariate statistical distributions Ø Is a measure of generalized variance of multiple variables If the determinant is positive, the matrix is called positive definiteà the matrix hasaninverse If the determinant is not positive, the matrix is called nonpositive definite à the matrix does not have an inverse EPSY 905: Matrix Algebra 47

48 WRAPPING UP EPSY 905: Matrix Algebra 48

49 Much Ado About Matrices Matrices show up nearly anytime multivariate statistics are used, often in the help/manual pages of the package you intend to use for analysis You don t have to do matrix algebra, but please do try to understand the concepts underlying matrices Your working with multivariate statistics will be better off because of even a small amount of understanding EPSY 905: Matrix Algebra 49

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