nyai - interactive machine learning by daniel hsu

Interactive machine learning

Daniel HsuColumbia University

Non-interactive machine learning

“Cartoon” of (non-interactive) machine learning

1. Get labeled data {(inputi , outputi )}ni=1.

2. Learn prediction function f̂ (e.g., classifier, regressor, policy)such that

f̂ (inputi ) ≈ outputi

for most i = 1, 2, . . . , n.

Goal: f̂ (input) ≈ output for future (input, output) pairs.Some applications: document classification, face detection,speech recognition, machine translation, credit rating, . . .

There is a lot of technology for doing this,and a lot of mathematical theories for understanding this.

“Cartoon” of (non-interactive) machine learning1. Get labeled data {(inputi , outputi )}ni=1.

for most i = 1, 2, . . . , n.

Goal: f̂ (input) ≈ output for future (input, output) pairs.

Some applications: document classification, face detection,speech recognition, machine translation, credit rating, . . .

for most i = 1, 2, . . . , n.

Interactive machine learning: example #1

Practicing physician

Loop:1. Patient arrives with symptoms, medical history, genome . . .2. Prescribe treatment.3. Observe impact on patient’s health (e.g., improves, worsens).

Goal prescribe treatments that yield good health outcomes.

Practicing physicianLoop:1. Patient arrives with symptoms, medical history, genome . . .

2. Prescribe treatment.3. Observe impact on patient’s health (e.g., improves, worsens).

Practicing physicianLoop:1. Patient arrives with symptoms, medical history, genome . . .2. Prescribe treatment.

3. Observe impact on patient’s health (e.g., improves, worsens).

Practicing physicianLoop:1. Patient arrives with symptoms, medical history, genome . . .2. Prescribe treatment.3. Observe impact on patient’s health (e.g., improves, worsens).

Website operator

Loop:1. User visits website with profile, browsing history . . .2. Choose content to display on website.3. Observe user reaction to content (e.g., click, “like”).

Goal choose content that yield desired user behavior.

Website operatorLoop:1. User visits website with profile, browsing history . . .

2. Choose content to display on website.3. Observe user reaction to content (e.g., click, “like”).

Website operatorLoop:1. User visits website with profile, browsing history . . .2. Choose content to display on website.

3. Observe user reaction to content (e.g., click, “like”).

Website operatorLoop:1. User visits website with profile, browsing history . . .2. Choose content to display on website.3. Observe user reaction to content (e.g., click, “like”).

E-mail service provider

Loop:1. Receive e-mail messages for users (spam or not).2. Ask users to provide labels for some borderline cases.3. Improve spam filter using newly labeled messages.

Goal maximize accuracy of spam filter,minimize number of queries to users.

E-mail service providerLoop:1. Receive e-mail messages for users (spam or not).

2. Ask users to provide labels for some borderline cases.3. Improve spam filter using newly labeled messages.

E-mail service providerLoop:1. Receive e-mail messages for users (spam or not).2. Ask users to provide labels for some borderline cases.

3. Improve spam filter using newly labeled messages.

E-mail service providerLoop:1. Receive e-mail messages for users (spam or not).2. Ask users to provide labels for some borderline cases.3. Improve spam filter using newly labeled messages.

Characteristics of interactive machine learning problems

1. Learning agent (a.k.a. “learner”) interacts with the world(e.g., patients, users) to gather data.

2. Learner’s performance based on learner’s decisions.3. Data available to learner depends on learner’s decisions.4.

This talk: two interactive machine learning problems,1. active learning, and2. contextual bandit learning.

(Our models for these problems have all but the last of above characteristics.)

2. Learner’s performance based on learner’s decisions.3. Data available to learner depends on learner’s decisions.4.

2. Learner’s performance based on learner’s decisions.

3. Data available to learner depends on learner’s decisions.4.

2. Learner’s performance based on learner’s decisions.3. Data available to learner depends on learner’s decisions.

2. Learner’s performance based on learner’s decisions.3. Data available to learner depends on learner’s decisions.4. State of the world depends on learner’s decisions.

State of theworld depends on learner’s decisions.

Active learning

Motivation

Recall non-interactive (a.k.a. passive) machine learning:

1. Get labeled data {(inputi , labeli )}ni=1.

2. Learn function f̂ (e.g., classifier, regressor, decision rule, policy)such that

f̂ (inputi ) ≈ labeli

for most i = 1, 2, . . . , n.

Problem: labels often much more expensive to get than inputs.E.g., can’t ask for spam/not-spam label of every e-mail.

Motivation

Recall non-interactive (a.k.a. passive) machine learning:

1. Get labeled data {(inputi , labeli )}ni=1.

2. Learn function f̂ (e.g., classifier, regressor, decision rule, policy)such that

f̂ (inputi ) ≈ labeli

for most i = 1, 2, . . . , n.

Problem: labels often much more expensive to get than inputs.E.g., can’t ask for spam/not-spam label of every e-mail.

Active learning

Basic structure of active learning:

I Start with pool of unlabeled inputs(and maybe some (input, label) pairs).

I Repeat:

1. Train prediction function f̂ using current labeled pairs.2. Pick some inputs from the pool, and query their labels.

I Final prediction function f̂ satisfies f̂ (input) ≈ label formost future (input, label) pairs.

I Number of label queries is small.(Can be selective/adaptive about which labels to query . . . )

Active learning

I Repeat:

1. Train prediction function f̂ using current labeled pairs.2. Pick some inputs from the pool, and query their labels.

Active learning

I Repeat:1. Train prediction function f̂ using current labeled pairs.

2. Pick some inputs from the pool, and query their labels.

Active learning

I Repeat:1. Train prediction function f̂ using current labeled pairs.2. Pick some inputs from the pool, and query their labels.

Active learning

Goal:I Final prediction function f̂ satisfies f̂ (input) ≈ label for

most future (input, label) pairs.

Active learning

Goal:I Final prediction function f̂ satisfies f̂ (input) ≈ label for

most future (input, label) pairs.

Sampling biasMain difficulty of active learning: sampling bias.

I At any point in active learning process, labeled data settypically not representative of overall data.

I Problem can derail learning and go unnoticed!

input ∈ R, label ∈ {0, 1}, classifier type = threshold functions.

45%45% 5%5%

Learner converges to classifier with 5% error rate, even thoughthere’s a classifier with 2.5% error rate. Not statistically consistent!

45%45% 5%5%

Sampling bias

Typical active learning heuristic:I Start with pool of unlabeled inputs.

I Pick a handful of inputs, and query their labels.I Repeat:

1. Train classifier f̂ using current labeled pairs.2. Pick input from pool closest to decision boundary of f̂ ,

and query its label.

45%45% 5%5%

Sampling bias

Typical active learning heuristic:I Start with pool of unlabeled inputs.I Pick a handful of inputs, and query their labels.

I Repeat:

45%45% 5%5%

Sampling bias

Typical active learning heuristic:I Start with pool of unlabeled inputs.I Pick a handful of inputs, and query their labels.I Repeat:

1. Train classifier f̂ using current labeled pairs.

2. Pick input from pool closest to decision boundary of f̂ ,and query its label.

45%45% 5%5%

Sampling bias

45%45% 5%5%

Sampling bias

45%45% 5%5%

Framework for statistically consistent active learning

Importance weighted active learning(Beygelzimer, Dasgupta, and Langford, ICML 2009)

I Start with unlabeled pool {inputi}ni=1.I For each i = 1, 2, . . . , n (in random order):

1. Get inputi .2. Pick probability pi ∈ [0, 1], toss coin with P(heads) = pi .3. If heads, query labeli , and include (inputi , labeli ) with

importance weight 1/pi in data set.

Importance weight = “inverse propensity” that labeli is queried.Used to account for sampling bias in error rate estimates.

I Start with unlabeled pool {inputi}ni=1.

I For each i = 1, 2, . . . , n (in random order):

1. Get inputi .

2. Pick probability pi ∈ [0, 1], toss coin with P(heads) = pi .3. If heads, query labeli , and include (inputi , labeli ) with

1. Get inputi .2. Pick probability pi ∈ [0, 1], toss coin with P(heads) = pi .

3. If heads, query labeli , and include (inputi , labeli ) withimportance weight 1/pi in data set.

Importance weighted active learning algorithms

How to choose pi?

1. Use your favorite heuristic (as long as pi is never too small).

2. Simple rule based on estimated error rate differences(Beygelzimer, Hsu, Langford, and Zhang, NIPS 2010).

In many cases, can prove the following:I Final classifier is as accurate as learner that queries all n labels.I Expected number of queries

∑ni=1 pi grows sub-linearly with n.

3. Solve a large convex optimization problem(Huang, Agarwal, Hsu, Langford, and Schapire, NIPS 2015).

Even stronger theoretical guarantees.

Latter two methods can be made to work with any classifier type,provided a good (non-interactive) learning algorithm.

How to choose pi?1. Use your favorite heuristic (as long as pi is never too small).

Contextual bandit learning

Protocol for contextual bandit learning:For round t = 1, 2, . . . ,T :

1. Observe contextt . [e.g., user profile]

2. Choose actiont ∈ Actions. [e.g., display ad]

3. Collect rewardt(actiont) ∈ [0, 1]. [e.g., click or no-click]

Goal: Choose actions that yield high reward.

Note: In round t, only observe reward for chosen actiont , and notreward that you would’ve received if you’d chosen a different action.

Protocol for contextual bandit learning:For round t = 1, 2, . . . ,T :

1. Observe contextt . [e.g., user profile]

Protocol for contextual bandit learning:For round t = 1, 2, . . . ,T :1. Observe contextt . [e.g., user profile]

Challenges in contextual bandit learning

1. Explore vs. exploit:I Should use what you’ve already learned about actions.I Must also learn about new actions that could be good.

2. Must use context effectively.I Which action is good likely to depend on current context.I Want to do as well as the best policy (i.e., decision rule)

π : context 7→ action

from some rich class of policies Π.

3. Selection bias, especially when exploiting.

Why is exploration necessary?

No-exploration approach:

1. Using historical data, learn a “reward predictor” for each actionbased on context:

r̂eward(action | context) .

2. Then deploy policy π̂, given by

π̂(context) := argmaxaction∈Actions

r̂eward(action | context) ,

and collect more data.

No-exploration approach:1. Using historical data, learn a “reward predictor” for each action

based on context:

No-exploration approach:1. Using historical data, learn a “reward predictor” for each action

based on context:

Using no-exploration

Example: two actions {a1, a2}, two contexts {x1, x2}.

Suppose initial policy says π̂(x1) = a1 and π̂(x2) = a2.

Observed rewardsa1 a2

x1 0.7 —x2 — 0.1

Reward estimatesa1 a2

x1 0.7 0.5x2 0.5 0.1

New policy: π̂′(x1) = π̂′(x2) = a1.

x1 0.7 —x2 0.3 0.1

x1 0.7 0.5x2 0.3 0.1

True rewardsa1 a2

x1 0.7 1.0x2 0.3 0.1

Never try a2 in context x1. Highly sub-optimal.

x1 0.7 —x2 — 0.1

x1 0.7 0.5x2 0.5 0.1

x1 0.7 —x2 0.3 0.1

x1 0.7 0.5x2 0.3 0.1

True rewardsa1 a2

x1 0.7 1.0x2 0.3 0.1

x1 0.7 —x2 — 0.1

x1 0.7 0.5x2 0.5 0.1

x1 0.7 —x2 0.3 0.1

x1 0.7 0.5x2 0.3 0.1

True rewardsa1 a2

x1 0.7 1.0x2 0.3 0.1

x1 0.7 —x2 — 0.1

x1 0.7 0.5x2 0.5 0.1

x1 0.7 —x2 0.3 0.1

x1 0.7 0.5x2 0.3 0.1

True rewardsa1 a2

x1 0.7 1.0x2 0.3 0.1

x1 0.7 —x2 — 0.1

x1 0.7 0.5x2 0.5 0.1

x1 0.7 —x2 0.3 0.1

x1 0.7 0.5x2 0.3 0.1

True rewardsa1 a2

x1 0.7 1.0x2 0.3 0.1

x1 0.7 —x2 — 0.1

x1 0.7 0.5x2 0.5 0.1

x1 0.7 —x2 0.3 0.1

x1 0.7 0.5x2 0.3 0.1

True rewardsa1 a2

x1 0.7 1.0x2 0.3 0.1

x1 0.7 —x2 — 0.1

x1 0.7 0.5x2 0.5 0.1

x1 0.7 —x2 0.3 0.1

x1 0.7 0.5x2 0.3 0.1

True rewardsa1 a2

x1 0.7 1.0x2 0.3 0.1

Never try a2 in context x1.

Highly sub-optimal.

x1 0.7 —x2 — 0.1

x1 0.7 0.5x2 0.5 0.1

x1 0.7 —x2 0.3 0.1

x1 0.7 0.5x2 0.3 0.1

True rewardsa1 a2

x1 0.7 1.0x2 0.3 0.1

Framework for simultaneous exploration and exploitation

I Initialize distribution W over some policies.

I For t = 1, 2, . . . ,T :

1. Observe contextt .2. Randomly pick policy π̂ ∼W ,

choose π̂(contextt) ∈ Actions.3. Collect rewardt(actiont) ∈ [0, 1],

update distribution W .

Can use “inverse propensity” importance weights to account forsampling bias when estimating expected rewards of policies.

I For t = 1, 2, . . . ,T :

1. Observe contextt .2. Randomly pick policy π̂ ∼W ,

I For t = 1, 2, . . . ,T :1. Observe contextt .

2. Randomly pick policy π̂ ∼W ,choose π̂(contextt) ∈ Actions.

3. Collect rewardt(actiont) ∈ [0, 1],update distribution W .

I For t = 1, 2, . . . ,T :1. Observe contextt .2. Randomly pick policy π̂ ∼W ,

choose π̂(contextt) ∈ Actions.

3. Collect rewardt(actiont) ∈ [0, 1],update distribution W .

Algorithms for contextual bandit learning

How to choose the distribution W over policies?

Exp4 (Auer, Cesa-Bianchi, Freund, and Schapire, FOCS 1995)

Maintains exponential weights over all policies in apolicy class Π.

Monster (Dudik, Hsu, Kale, Karampatziakis, Langford, Reyzin, andZhang, UAI 2011)

Solves optimization problem to come up withpoly(T , log |Π|)-sparse weights over policies.

Mini-monster (Agarwal, Hsu, Kale, Langford, Li, and Schapire, ICML 2014)

Simpler and faster algorithm to solve optimizationproblem; get much sparser weights over policies.

Latter two methods rely on a good (non-interactive) learningalgorithm for policy class Π.

How to choose the distribution W over policies?Exp4 (Auer, Cesa-Bianchi, Freund, and Schapire, FOCS 1995)

Wrap-up

I Interactive machine learning models better capture howmachine learning is used in many applications (thannon-interactive frameworks).

I Sampling bias is a pervasive issue.

I Research: how to use non-interactive machine learningtechnology in these new interactive settings.

Thanks!

Wrap-up

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Wrap-up

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Wrap-up

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Wrap-up

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nyai - interactive machine learning by daniel hsu

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