Week 11

11: Machine Learning System Design

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Machine learning systems design

  • In this section we'll touch on how to put together a system
  • Previous sections have looked at a wide range of different issues in significant focus
  • This section is less mathematical, but material will be very useful non-the-less
    • Consider the system approach
    • You can understand all the algorithms, but if you don't understand how to make them work in a complete system that's no good!

Prioritizing what to work on - spam classification example

  • The idea of prioritizing what to work on is perhaps the most important skill programmers typically need to develop
    • It's so easy to have many ideas you want to work on, and as a result do none of them well, because doing one well is harder than doing six superficially
      • So you need to make sure you complete projects
      • Get something "shipped" - even if it doesn't have all the bells and whistles, that final 20% getting it ready is often the toughest
      • If you only release when you're totally happy you rarely get practice doing that final 20%
    • So, back to machine learning...
  • Building a spam classifier
  • Spam is email advertising
  • What kind of features might we define
    • Spam (1)
      • Misspelled word
    • Not spam (0)
      • Real content
  • How do we build a classifier to distinguish between the two
    • Feature representation
      • How do represent x (features of the email)?
        • y = spam (1) or not spam (0)

One approach - choosing your own features

  • Chose 100 words which are indicative of an email being spam or not spam
    • Spam --> e.g. buy, discount, deal
    • Non spam --> Andrew, now
    • All these words go into one long vector
  • Encode this into a reference vector
    • See which words appear in a message
  • Define a feature vector x
    • Which is 0 or 1 if a word corresponding word in the reference vector is present or not
      • This is a bitmap of the word content of your email
    • i.e. don't recount if a word appears more than once
    • In practice its more common to have a training set and pick the most frequently n words, where n is 10 000 to 50 000
      • So here you're not specifically choosing your own features, but you are choosing how you select them from the training set data

What's the best use of your time to improve system accuracy?

  • Natural inclination is to collect lots of data
    • Honey pot anti-spam projects try and get fake email addresses into spammers' hands, collect loads of spam
    • This doesn't always help though
  • Develop sophisticated features based on email routing information (contained in email header)
    • Spammers often try and obscure origins of email
    • Send through unusual routes
  • Develop sophisticated features for message body analysis
    • Discount == discounts?
    • DEAL == deal?
  • Develop sophisticated algorithm to detect misspelling
    • Spammers use misspelled word to get around detection systems
  • Often a research group randomly focus on one option
    • May not be the most fruitful way to spend your time
    • If you brainstorm a set of options this is really good
      • Very tempting to just try something

Error analysis

  • When faced with a ML problem lots of ideas of how to improve a problem
    • Talk about error analysis - how to better make decisions
  • If you're building a machine learning system often good to start by building a simple algorithm which you can implement quickly
    • Spend at most 24 hours developing an initially bootstrapped algorithm
      • Implement and test on cross validation data
    • Plot learning curves to decide if more data, features etc will help algorithmic optimization
      • Hard to tell in advance what is important
      • Learning curves really help with this
      • Way of avoiding premature optimization
        • We should let evidence guide decision making regarding development trajectory
    • Error analysis
      • Manually examine the samples (in cross validation set) that your algorithm made errors on
      • See if you can work out why
        • Systematic patterns - help design new features to avoid these shortcomings
      • e.g.
        • Built a spam classifier with 500 examples in CV set
          • Here, error rate is high - gets 100 wrong
        • Manually look at 100 and categorize them depending on features
          • e.g. type of email
        • Looking at those email
          • May find most common type of spam emails are pharmacy emails, phishing emails
            • See which type is most common - focus your work on those ones
          • What features would have helped classify them correctly
            • e.g. deliberate misspelling
            • Unusual email routing
            • Unusual punctuation
            • May fine some "spammer technique" is causing a lot of your misses
              • Guide a way around it
    • Importance of numerical evaluation
      • Have a way of numerically evaluated the algorithm
      • If you're developing an algorithm, it's really good to have some performance calculation which gives a single real number to tell you how well its doing
      • e.g.
        • Say were deciding if we should treat a set of similar words as the same word
        • This is done by stemming in NLP (e.g. "Porter stemmer" looks at the etymological stem of a word)
        • This may make your algorithm better or worse
          • Also worth consider weighting error (false positive vs. false negative)
            • e.g. is a false positive really bad, or is it worth have a few of one to improve performance a lot
        • Can use numerical evaluation to compare the changes
          • See if a change improves an algorithm or not
      • A single real number may be hard/complicated to compute
        • But makes it much easier to evaluate how changes impact your algorithm
    • You should do error analysis on the cross validation set instead of the test set


Error metrics for skewed analysis

  • Once case where it's hard to come up with good error metric - skewed classes
  • Example
    • Cancer classification
      • Train logistic regression model hθ(x) where
        • Cancer means y = 1
        • Otherwise y = 0
      • Test classifier on test set
        • Get 1% error 
          • So this looks pretty good..
        • But only 0.5% have cancer
          • Now, 1% error looks very bad!
    • So when one number of examples is very small this is an example of skewed classes
      • LOTS more of one class than another
      • So standard error metrics aren't so good
  • Another example
    • Algorithm has 99.2% accuracy 
    • Make a change, now get 99.5% accuracy
      • Does this really represent an improvement to the algorithm?
    • Did we do something useful, or did we just create something which predicts y = 0 more often
      • Get very low error, but classifier is still not great

Precision and recall

  • Two new metrics - precision and recall
    • Both give a value between 0 and 1
    • Evaluating classifier on a test set
    • For a test set, the actual class is 1 or 0
    • Algorithm predicts some value for class, predicting a value for each example in the test set
      • Considering this, classification can be
        • True positive (we guessed 1, it was 1)
        • False positive (we guessed 1, it was 0)
        • True negative (we guessed 0, it was 0)
        • False negative (we guessed 0, it was 1)
    • Precision
      • How often does our algorithm cause a false alarm?
      • Of all patients we predicted have cancer, what fraction of them actually have cancer
        • = true positives / # predicted positive
        • = true positives / (true positive + false positive)
      • High precision is good (i.e. closer to 1)
        • You want a big number, because you want false positive to be as close to 0 as possible
    • Recall
      • How sensitive is our algorithm?
      • Of all patients in set that actually have cancer, what fraction did we correctly detect
        • = true positives / # actual positives
        • = true positive / (true positive + false negative)
      • High recall is good (i.e. closer to 1)
        • You want a big number, because you want false negative to be as close to 0 as possible
    • By computing precision and recall get a better sense of how an algorithm is doing
      • This can't really be gamed
      • Means we're much more sure that an algorithm is good
    • Typically we say the presence of a rare class is what we're trying to determine (e.g. positive (1) is the existence of the rare thing)

Trading off precision and recall

  • For many applications we want to control the trade-off between precision and recall
  • Example
    • Trained a logistic regression classifier 
      • Predict 1 if hθ(x) >= 0.5
      • Predict 0 if hθ(x) < 0.5
    • This classifier may give some value for precision and some value for recall
    • Predict 1 only if very confident
      • One way to do this modify the algorithm we could modify the prediction threshold
        • Predict 1 if hθ(x) >= 0.8
        • Predict 0 if hθ(x) < 0.2
      • Now we can be more confident a 1 is a true positive
      • But classifier has lower recall - predict y = 1 for a smaller number of patients
        • Risk of false negatives
    • Another example - avoid false negatives
      • This is probably worse for the cancer example
        • Now we may set to a lower threshold
          • Predict 1 if hθ(x) >= 0.3
            • Predict 0 if hθ(x) < 0.7
        • i.e. 30% chance they have cancer
        • So now we have have a higher recall, but lower precision
          • Risk of false positives, because we're less discriminating in deciding what means the person has cancer
  • This threshold defines the trade-off
    • We can show this graphically by plotting precision vs. recall
    • This curve can take many different shapes depending on classifier details
    • Is there a way to automatically chose the threshold
      • Or, if we have a few algorithms, how do we compare different algorithms or parameter sets?
    • How do we decide which of these algorithms is best?
      • We spoke previously about using a single real number evaluation metric
      • By switching to precision/recall we have two numbers
      • Now comparison becomes harder 
        • Better to have just one number 
      • How can we convert P & R into one number?
        • One option is the average - (P + R)/2
          • This is not such a good solution 
            • Means if we have a classifier which predicts y = 1 all the time you get a high recall and low precision
            • Similarly, if we predict Y rarely get high precision and low recall
            • So averages here would be 0.45, 0.4 and 0.51
              • 0.51 is best, despite having a recall of 1 - i.e. predict y=1 for everything
          • So average isn't great
        • F1Score (fscore)
          • = 2 * (PR/ [P + R])
          • Fscore is like taking the average of precision and recall giving a higher weight to the lower value
        • Many formulas for computing comparable precision/accuracy values
          • If P = 0 or R = 0 the Fscore = 0
          • If P = 1 and R = 1 then Fscore = 1
          • The remaining values lie between 0 and 1
  • Threshold offers a way to control trade-off between precision and recall
  • Fscore gives a single real number evaluation metric
    • If you're trying to automatically set the threshold, one way is to try a range of threshold values and evaluate them on your cross validation set 
      • Then pick the threshold which gives the best fscore.

Data for machine learning

  • Now switch tracks and look at how much data to train on
  • On early videos caution on just blindly getting more data
    • Turns out under certain conditions getting more data is a very effective way to improve performance

Designing a high accuracy learning system

  • There have been studies of using different algorithms on data
    • Data - confusing words (e.g. two, to or too)
    • Algorithms
      • Perceptron (logistic regression)
      • Winnow
        • Like logistic regression
        • Used less now
      • Memory based
        • Used less now
        • Talk about this later
      • Naive Bayes
        • Cover later
    • Varied training set size and tried algorithms on a range of sizes
    • What can we conclude
      • Algorithms give remarkably similar performance
      • As training set sizes increases accuracy increases
      • Take an algorithm, give it more data, should beat a "better" one with less data
      • Shows that
        • Algorithm choice is pretty similar
        • More data helps
  • When is this true and when is it not?
    • If we can correctly assume that features have enough information to predict y accurately, then more data will probably help
      • A useful test to determine if this is true can be, "given x, can a human expert predict y?"
    • So lets say we use a learning algorithm with many parameters such as logistic regression or linear regression with many features, or neural networks with many hidden features
      • These are powerful learning algorithms with many parameters which can fit complex functions 
        • Such algorithms are low bias algorithms
          • Little systemic bias in their description - flexible
      • Use a small training set
        • Training error should be small
      • Use a very large training set
        • If the training set error is close to the test set error 
        • Unlikely to over fit with our complex algorithms
        • So the test set error should also be small
    • Another way to think about this is we want our algorithm to have low bias and low variance
      • Low bias --> use complex algorithm
      • Low variance --> use large training set
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