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Corpora and Statistical Methods
Albert Gatt
Probability distributions
Example 1: Book publishing
 Case:
 publishing house considers whether to publish a new textbook
on statistical NLP
 considerations include: production cost, expected sales, net
profits (given cost)
 Problem:
 to publish or not to publish?
 depends on expected sales and profits
 if published, how many copies?
 depends on demand and cost
Example 1: Demand & cost figures
 Suppose:
 book costs €35, of which:
 publisher gets €25
 bookstore gets €6
 author gets €4
 To make a decision, publisher needs to estimate profits as a
function of the probability of selling n books, for different
values of n.
 profit = (€25 * n) – overall production cost
Terminology
 Random variable
 In this example, the expected profit from selling n books is our
random variable
 It takes on different values, depending on n
 We use uppercase (e.g. X) to denote the random variable
 Distribution
 The different values of X (denoted x) form a distribution.
 If each value x can be assigned a probability (the probability of making
a given profit), then we can plot each value x against its likelihood.
Definitions
 Random variable
 A variable whose numerical value is determined by chance. Formally, a
function that returns a unique numerical value determined by the outcome
of an uncertain situation.
 Can be discrete (our exclusive focus) or continuous
 Probability distribution
 For a discrete random variable X, the probability distribution p(x) gives the
probabilities for each value x of X.
 The probabilities p(x) of all possible values of X sum to 1.
 The distribution tells us how much out of the overall probability space (the
“probability mass”), each value of x takes up.
Tabulated probability distribution
No. copies sold
Prod. cost
Profits
(X)
Probability
P(x)
5,000
£275,000
-£150,000
.20
10,000
£300,000
-£50,000
.40
20,000
£350,000
£150,000
.25
30,000
£400,000
£350,000
.10
40,000
£450,000
£550,000
.05
Plotting the distribution
Uses of a probability distribution
 Computation of:
 mean: the expected value of X in the long run
 based on the specific values of X, and their probability
 NB: NOT interpreted as value in a sample of data, but expected
(future) value based on sample.
 standard deviation & variance: the extent to which actual values of X
will differ from the mean
 skewness: the extent to which our distribution is “balanced”, i.e.
whether it’s symmetrical
In graphics…
Mean: expected value
in the long run
SD & variance:
How much actual values
deviate from mean overall
Skewness:
Symmetry or “tail”
of our distribution
Measures of expectation and variation
The expected value (mean)
 The expected value of a discrete random variable X, denoted
E[X] or μ, is a weighted average of the values of X
 weighted, because not all values x will have the same probability
 estimated by summing, for all values of X, the product of x and its
probability p(x)
  E[ X ] 
 xp( x)
x
More on expected value
 The mean or expected value tells us that, in the long run, we
can expect X to have the value μ.
 E.g. in our example, our book publisher can expect longterm profits of:
(-150,000 * .2) + (-50,000 * .4) + (150,000 * .25) +
(350,000 * .1) +
(550,000 * .05)
= €50,000
Variance
 Mean is the expected value of X, E[X]
 Variance (σ2) reflects the extent to which the actual
outcomes deviate from expectation (i.e. from E[X])
 σ2 = E[(X – μ)2] = Σ(x – μ)2p(x)
 i.e. the weighted sum of deviations squared
 Reasons for squaring:
 eliminates the distinction between +ve and –ve
 makes it exponential: larger deviations are given more importance
 e.g. one deviation of 10 is as large as 4 deviations of 5
Standard deviation
 Variance gives overall dispersion or variation
 Standard deviation (σ) is the dispersion of possible outcomes;
it indicates how spread out the distribution is.
 estimated as square root of variance
  2 
2
(
x


)
p( x)

x
The book publishing example again
 Recall that for our new book on stat NLP, expected profit is
£50,000
 What’s the standard deviation?
 need to estimate (50000-x)2 for all x
 multiply by p(x) in each case
 take the square root of the result
 This is left as an exercise…
Skewness
 The mean gives us the “centre” of a distribution.
 Standard deviation gives us dispersion.
 Skewness (denoted γ “gamma”) is a measure of the symmetry
of the outcomes.

 (x  )
x

3
3
Skewness, continued
 The formula calculates the average value of cubed deviations by the standard
deviation cubed.
 Why cubed?
 The cube of a positive deviation is itself positive; that of a negative is itself
negative. We want both, as we want to know deviations both to the left (-ve)
and right (+ve) of the mean.
 Like the variance estimation, this emphasises large deviations in either
direction (it’s exponential).
 If the outcomes are symmetrical around the mean, then +ve and –ve
deviations are balanced, and skewness is 0.
Graphical display of skewness
Positive skewness:
tail going right
Negative skewness:
tail going left
Skewness and language
 By Zipf’s law (next week), word frequencies do not cluster
around the mean.
 There are a few highly frequent words (making up a large proportion
of overall word frequency)
 There are many highly infrequent words (f = 1 or f = 2)
 So the Zipfian distribution is highly skewed.
 We will hear more on the Zipfian distribution in the next lecture.
The concept of information
What is information?
 Main ingredient:
 an information source, which “transmits” symbols from a finite
alphabet S
 every symbol is denoted si
 we call a sequence of such symbols a text
 assume a probability distribution s.t. every si has probability p(si)
 Example:
 a dice is an information source; every throw yields a symbol from the
alphabet {1,2,3,4,5,6}
 6 successive throws yield a text of 6 symbols
Quantifying information
 Intuition:
 the more probable a symbol is, the less information it yields
 “something seen very often is not very surprising”
 So information is the inverse probability of the symbol
I ( si )  log b
1
  log p( si )
p ( si )
 for some b > 1. Usually we use base 2
 Another term for I(s) is surprisal
Properties of I
1.
Non-negative
2.
If p(s) = 1, I(s) = 0
3.
If 2 events s1, s2 are independent, then:
I (si , s2 )  I ( p(s1 )  p(s2 ))  I (s1 )  I (s2 )
4.
Monotonic: slight changes in probability result in slight
changes in I
Aggregate measure of information

What is the information content of a text (sequence of
symbols)?


1.
this is the same as finding the average information of a random
variable
the measure is called Entropy, denoted H
Define X as a random variable over the symbols in our
alphabet
P(s) = P(X=s) for all s in our alphabet
2.
Estimate H(P)
Entropy
 The entropy (or information) of a probability distribution is
1
H b ( P)   P( s) log b
  P( s) log b P( s)
P( s )
sS
sS
 entropy is the expected value (mean) of the surprisal
 the value is interpreted as the number of “bits” of information
Entropy example
 Source = an 8-sided die
 Alphabet S = {1,2,3,4,5,6,7,8}
 every si has p = 1/8
8
8
1
1
1
H ( P)   p ( si ) log p ( si )   log   log  log 8  3
8
8
i 1
i 1 8
Interpretation of entropy
 The information contained in the distribution P (the more
unpredictable the outcomes, the higher the entropy)
 The message length if the message was generated according to
P and coded optimally
Interpretation cont/d
 For the 8-sided die example, the result H(P)=3 tells us we need 3
bits on average to “transmit” the result of rolling an 8-sided die:
1
2
3
4
5
6
7
8
001 010 011 100 101 110 111 000
 We can’t do it in less than 3 bits
Entropy for multiple variables
 So far we have dealt with a single random variable
 The joint entropy of a pair of RVs:
1
H ( X , Y )   P( x, y ) log b
P ( x, y )
xX yY
   P( x, y ) log b P( x, y )
xX yY
Conditional Entropy
 Given X and Y, how much information about Y do we gain if we
know X?
 a version of entropy using conditional probability: H(Y|X)
H (Y | X )

 P( x) H (Y | X
x X


 x)


P ( x)   P ( y | x) log P ( y | x)

x X
 yY

   P ( x) P ( y | x) log P ( y | x)
x X yY
Mutual information
Mutual information
 Just as probability can change based on posterior knowledge,
so can information.
 Suppose our distribution gives us the probability P(a) of
observing the symbol a.
 Suppose we first observe the symbol b.
 If a and b are not independent, this should alter our
information state with respect to the probability of observing
a.
 i.e. we can compute p(a|b)
Mutual info between two symbols
 The change in our information about a on observing b is:
 1 
 1 
  log

I (a; b)  log
 P(a ) 
 P ( a | b) 
 P ( a | b) 

 log
 P(a) 
 If a and b are completely independent, I(a;b)=0.
Averaging mutual information
 We want to average mutual information between all values of a
random variable A and those of a random variable B.
 P ( a | b)  I ( a ; b)
I ( A; b) 
i
i


i
i
 P ( ai | b ) 

P(ai | b)  log

 P ( ai ) 
 And similarly:
I (a; B) 

j
 P(bi | a) 

P(a | b j )  log
 P(b j ) 


Combining the two…
I ( A; B) 
 P(a )  I (a ; B)
i
i
i
 P ( ai , b j ) 


P(ai , b j )  log
 P(ai ) P(b j ) 
i
j


 I ( B; A)

 Thus, mutual info involves taking the joint probability and
dividing by the individual probabilities.
 I.e. a comparison of the likelihood of observing a, b together
vs. separately.
Mutual Information: summary
 Gives a measure of reduction in uncertainty about a random
variable X, given knowledge of Y
 quantifies how much information about X is contained in Y
Some more on I(X;Y)
 In statistical NLP, we often calculate pointwise mutual
information
 this is the mutual information between two points on a
distribution
 I(x;y) rather than I(X;Y)
 used for some applications in lexical acquisition
Mutual Information -- example
 Suppose we’re interested in the collocational strength of two
words x and y
 e.g. bread and butter
 mutual information quantifies the likelihood of observing x and y
together (in some window)
 If there is no interesting relationship, knowing about bread
tells us nothing about the likelihood of encountering butter
 Here, P(x,y) = P(x)P(y) and I(x;y) = 0
 This is the Church and Hanks (1991) approach.
 NB. The approach uses pointwise MI