Machine Learning in Practice Lecture 5 Carolyn Penstein Rosé Language Technologies Institute/ Human-Computer Interaction Institute

Plan for the Day Announcements  Assignment 3  Project update Quiz 2 Naïve Bayes

Decision Tables vs Decision Trees Open World Assumption  Only examine some attributes in particular contexts  Uses majority class within a context to eliminate closed world requirement Divide and Conquer approach Closed World Assumption  Every case enumerated No generalization except by limiting number of attributes 1R algorithms produces the simplest possible Decision Table

Decision Tables vs Decision Trees Open World Assumption  Only examine some attributes in particular contexts  Uses majority class within a context to eliminate closed world requirement Divide and Conquer approach Closed World Assumption  Every case enumerated No generalization except by limiting number of attributes 1R algorithms produces the simplest possible Decision Table

Decision Tables vs Decision Trees Open World Assumption  Only examine some attributes in particular contexts  Uses majority class within a context to eliminate closed world requirement Divide and Conquer approach Closed World Assumption  Every case enumerated No generalization except by limiting number of attributes 1R algorithms produces the simplest possible Decision Table

Decision Tables vs Decision Trees Open World Assumption  Only examine some attributes in particular contexts  Uses majority class within a context to eliminate closed world requirement Divide and Conquer approach Closed World Assumption  Every case enumerated No generalization except by limiting number of attributes 1R algorithms produces the simplest possible Decision Table

Decision Tables vs Decision Trees Open World Assumption  Only examine some attributes in particular contexts  Uses majority class within a context to eliminate closed world requirement Divide and Conquer approach Closed World Assumption  Every case enumerated No generalization except by limiting number of attributes 1R algorithms produces the simplest possible Decision Table

Decision Tables vs Decision Trees Open World Assumption  Only examine some attributes in particular contexts  Uses majority class within a context to eliminate closed world requirement Divide and Conquer approach Closed World Assumption  Every case enumerated No generalization except by limiting number of attributes 1R algorithms produces the simplest possible Decision Table

Weights Versus Probabilities: A Historical Perspective Artificial intelligence is about separating declarative and procedural knowledge Algorithms can reason using knowledge in the form of rules  E.g., expert systems, some cognitive models This can be used for planning, diagnosing, inferring, etc.

Weights Versus Probabilities: A Historical Perspective But what about reasoning under uncertainty?  Incomplete knowledge  Errors  Knowledge with exceptions  A changing world

Rules with Confidence Values Will Carolyn eat the chocolate?  Positive evidence Carolyn usually eats what she likes. (.85) Carolyn likes chocolate. (.98)  Negative Evidence Carolyn doesn’t normally eat more than one dessert per day. (.75) Carolyn already drank hot chocolate. (.95) Hot chocolate is sort of like a dessert. (.5) How do you combine positive and negative evidence?

What is a probability? You have a notion of an event  Tossing a coin How many things can happen  Heads, tails How likely are you to get heads on a random toss?  50% Probabilities give you a principled way of combining predictions  How likely are you to get heads twice in a row? .5 *.5 =.25

Statistical Modeling Basics Rule and tree based methods use contingencies between patterns of attribute values as a basis for decision making Statistical models treat attributes as independent pieces of evidence that the decision should go one way or another Most of the time in real data sets the values of the different attributes are not independent of each other

Statistical Modeling Pros and Cons Statistical modeling people argue that statistical models are more elegant than other types of learned models because of their formal properties  You can combine probabilities in a principled way You can also combine the “weights” that other approaches assign But it is more ad hoc

Statistical Modeling Pros and Cons Statistical approach depends on assumptions that are not in general true In practice statistical approaches don’t work better than “ad-hoc” methods

Statistical Modeling Basics Even without features you can make a prediction about a class based on prior probabilities  You would always predict the majority class

Statistical Modeling Basics Statistical approaches balance evidence from features with prior probabilities  Thousand feet view: Can I beat performance based on priors with performance including evidence from features?  On very skewed data sets it can be hard to beat your priors (evaluation done based on percent correct)

Basic Probability If you roll a pair of dice, what is the probability that you will get a 4 and a 5?

Basic Probability If you roll a pair of dice, what is the probability that you will get a 4 and a 5? 1/18

Basic Probability If you roll a pair of dice, what is the probability that you will get a 4 and a 5? 1/18 How did you figure it out?

Basic Probability If you roll a pair of dice, what is the probability that you will get a 4 and a 5? 1/18 How did you figure it out? How many ways can the dice land?

Basic Probability If you roll a pair of dice, what is the probability that you will get a 4 and a 5? 1/18 How did you figure it out? How many ways can the dice land? How many of these satisfy our constraints?

Basic Probability If you roll a pair of dice, what is the probability that you will get a 4 and a 5? 1/18 How did you figure it out? How many ways can the dice land? How many of these satisfy our constraints? Divide ways to satisfy constraints by number of things that can happen

Basic Probability What if you want the first die to be 5 and the second die to be 4?

Basic Probability What if you want the first die to be 5 and the second die to be 4? What if you know the first die landed on 5?

Computing Conditional Probabilities

What is the probability of high humidity?

Computing Conditional Probabilities What is the probability of high humidity? What is the probability of high humidity given that the temperature is cool?

How do we train a model?

For every value of every feature, store a count. How many times do you see Outlook = rainy?

How do we train a model? For every value of every feature, store a count. How many times do you see Outlook = rainy? What is P(Outlook = rainy)?

How do we train a model? We also need to know what evidence each value of every feature gives of each possible prediction (or how typical it would be for instances of that class) What is P(Outlook = rainy | Class = yes)?

How do we train a model? We also need to know what evidence each value of every feature gives of each possible prediction (or how typical it would be for instances of that class) What is P(Outlook = rainy | Class = yes)? Store counts on (class value, feature value) pairs How many times is Outlook = rainy when class = yes?

How do we train a model? We also need to know what evidence each value of every feature gives of each possible prediction (or how typical it would be for instances of that class) What is P(Outlook = rainy | Class = yes)? Store counts on (class value, feature value) pairs How many times is Outlook = rainy when class = yes? Likelihood that play = yes if Outlook = rainy = Count(yes & rainy)/ Count(yes) * Count(yes)/Count(yes or no)

How do we train a model? Now try to compute likelihood play = yes for Outlook = overcast, Temperature = hot, Humidity = high, Windy = FALSE

Combinations of features? E.g., P(play = yes | Outlook = rainy & Temperature = hot) Multiply conditional probabilities for each predictor and prior probability of predicted class together before you normalize  P(play = yes | Outlook = rainy & Temperature = hot)  Likelihood of yes = Count(yes & rainy)/ Count(yes) * Count(yes & hot)/ Count(yes) * Count(yes)/Count(yes or no)  After you compute the likelihood of yes and likelihood of no, you will normalize to get probability of yes and probability of no

Unknown Values Not a problem for Naïve Bayes Probabilities computed using only the specified values Likelihood that play = yes when Outlook = sunny, Temperature = cool, Humidity = high, Windy = true  2/9 * 3/9 * 3/9 * 3/9 * 9/14  If Outlook is unknown, 3/9 * 3/9 * 3/9 * 9/14 Likelihoods will be higher when there are unknown values  Factored out during normalization

Numeric Values List values of numeric feature for all class features  Values for play = yes: 83, 70,68, 64, 69, 75, 75, 72, 81 Compute Mean and Standard Deviation  Values for play = yes: 83, 70, 68, 64, 69,75, 75, 72, 81  = 73,  = 6.16  Values for play = no: 85, 80, 65, 72, 71  = 74.6,  = 7.89 Compute likelihoods  f(x) = [1/sqrt(2   )]e (x-  ) 2 /2  2 Normalize using proportion of predicted class feature as before

Bayes Theorem How would you compute the likelihood that a person was a bagpipe major given that they had red hair?

Bayes Theorem How would you compute the likelihood that a person was a bagpipe major given that they had red hair? Could you compute the likelihood that a person has red hair given that they were a bagpipe major?

Bayes Theorem How would you compute the likelihood that a person was a bagpipe major given that they had red hair? Could you compute the likelihood that a person has red hair given that they were a bagpipe major?

Another Example ice-cream {chocolate, vanilla, coffee, rocky-road, cake {chocolate, yummy chocolate,chocolate,yum vanilla,chocolate,good coffee,chocolate,yum coffee,vanilla,ok rocky-road,chocolate,yum is-yummy Compute conditional probabilities for each attribute value/class pair  P(B|A) = Count(B&A)/Count(A)  P(coffee ice-cream | yum) =.25  P(vanilla ice-cream | yum) = 0

Another Example ice-cream {chocolate, vanilla, coffee, rocky-road, cake {chocolate, yummy chocolate,chocolate,yum vanilla,chocolate,good coffee,chocolate,yum coffee,vanilla,ok rocky-road,chocolate,yum is-yummy  What class would you assign to strawberry ice cream with chocolate cake?  Compute likelihoods and then normalize  Note: this model cannot take into account that the class might depend on how well the cake and ice cream “go together” What is the likelihood that the answer is yum? P(strawberry|yum) =.25 P(chocolate cake|yum) = *.75 *.66 =.124 What is the likelihood that The answer is good? P(strawberry|good) = 0 P(chocolate cake|good) = 1 0* 1 *.17 = 0 What is the likelihood that The answer is ok? P(strawberry|ok) = 0 P(chocolate cake|ok) = 0 0*0 *.17 = 0

Another Example ice-cream {chocolate, vanilla, coffee, rocky-road, cake {chocolate, yummy chocolate,chocolate,yum vanilla,chocolate,good coffee,chocolate,yum coffee,vanilla,ok rocky-road,chocolate,yum is-yummy What about vanilla ice cream and vanilla cake Intuitively, there is more evidence that the selected category should be Good. What is the likelihood that the answer is yum? P(vanilla|yum) = 0 P(vanilla cake|yum) =.25 0*.25 *.66= 0 What is the likelihood that The answer is good? P(vanilla|good) = 1 P(vanilla cake|good) = 0 1*0 *.17= 0 What is the likelihood that The answer is ok? P(vanilla|ok) = 0 P(vanilla cake|ok) = 1 0* 1 *.17 = 0

Statistical Modeling with Small Datasets When you train your model, how many probabilities are you trying to estimate? This statistical modeling approach has problems with small datasets where not every class is observed in combination with every attribute value  What potential problem occurs when you never observe coffee ice-cream with class ok?  When is this not a problem?

Smoothing One way to compensate for 0 counts is to add 1 to every count Then you never have 0 probabilities But what might be the problem you still have on small data sets?

Naïve Bayes with ice-cream {chocolate, vanilla, coffee, rocky-road, cake {chocolate, yummy chocolate,chocolate,yum vanilla,chocolate,good coffee,chocolate,yum coffee,vanilla,ok rocky-road,chocolate,yum is-yummy What is the likelihood that the answer is yum? P(vanilla|yum) =.11 P(vanilla cake|yum) =.33.11*.33*.66=.03 What is the likelihood that The answer is good? P(vanilla|good) =.33 P(vanilla cake|good) = *.33 *.17 =.02 What is the likelihood that The answer is ok? P(vanilla|ok) =.17 P(vanilla cake|ok) = *.66 *.17 =.02

Take Home Message Naïve Bayes is a simple form of statistical machine learning It’s naïve in that it assumes that all attributes are independent In the training process, counts are kept that indicate the connection between attribute values and predicted class values 0 counts interfere with making predictions, but smoothing can help address this difficulty