Alternative Forecasting Methods: Bootstrapping

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Alternative Forecasting Methods Bootstrapping

• Bryce Bucknell
• Jim Burke
• Ken Flores
• Tim Metts

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Agenda
Scenario
Obstacles
Regression Model
Bootstrapping
Applications and Uses
Results
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Scenario
You have been recently hired as the statistician
for the University of Notre Dame football team.
You are tasked with performing a statistical
analysis for the first year of the Charlie Weis
era. Specifically, you have been asked to
develop a regression model that explains the
relationship between key statistical categories
and the number of points scored by the offense.
You have a limited number of data points, so you
must also find a way to ensure that the
regression results generated by the model are
reliable and significant.
Problems/Obstacles

• Central Limit Theorem
• Replication of data
• Sampling
• Variance of error terms

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Constrained by the Central Limit Theorem
In selecting simple random samples of size n from
a population, the sampling distribution of the
sample mean x can be approximated by a normal
probability distribution as the sample size
becomes large. It is generally accepted that the
sample size must be 30 or greater to satisfy the
large-sample condition of the theorem.
_
Sample N 1
Sample N 2
Sample N 3
Sample N 4
1. http//www.statisticalengineering.com/central_l
imit_theorem_(summary).htm
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Central Limit Theorem
Central Limit theorem is the foundation for many
statistical procedures, because the distribution
of the phenomenon under study does NOT have to be
Normal because its average WILL tend to be
normal.
Why is the assumption of a normal distribution
important?

• A normal distribution allows for the application
  of the empirical rule 68, 95 and 99.7
• Chebyshevs Theorem no more than 1/4 of the
  values are more than 2 standard deviations away
  from the mean, no more than 1/9 are more than 3
  standard deviations away, no more than 1/25 are
  more than 5 standard deviations away, and so on.
• The assumption of a normally distributed data
  allows descriptive statistics to be used to
  explain the nature of the population

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Not enough data available?
Monte Carlo simulation, a type of spreadsheet
simulation, is used to randomly generate values
for uncertain variables over and over to simulate
a model.

• Monte Carlo methods randomly select values to
  create scenarios
• The random selection process is repeated many
  times to create multiple scenarios
• Through the random selection process, the
  scenarios give a range of possible solutions,
  some of which are more probable and some less
  probable
• As the process is repeated multiple times, 10,000
  or more, the average solution will give an
  approximate answer to the problem
• The accuracy can be improved by increasing the
  number of scenarios selected

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Sampling without Replacement
Simple Random Sampling

• A simple random sample from a population is a
  sample chosen randomly, so that each possible
  sample has the same probability of being chosen.
• In small populations such sampling is typically
  done "without replacement
• Sampling without replacement results in
  deliberate avoidance of choosing any member of
  the population more than once
• This process should be used when outcomes are
  mutually exclusive, i.e. poker hands

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Sampling with Replacement

• Initial data set is not sufficiently large enough
  to use simple random sampling without replacement
• Through Monte Carlo simulation we have been able
  to replicate the original population
• Units are sampled from the population one at a
  time, with each unit being replaced before the
  next is sampled.
• One outcome does not affect the other outcomes
• Allows a greater number of potential outcomes
  than sampling without replacement
• If observations were not replaced there would not
  be enough independent observations to create a
  sample size of n 30

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Hetroscedasticity vs. Homoscedasticity
Homoscedasticity constant variance
Hetroscedasticity nonconstant variance

• All random variables have the same finite
  variance
• Simplifies mathematical and computational
  treatment
• Leads to good estimation results in data mining
  and regression

• Random variables may have different variances
• Standard errors of regression coefficients may be
  understated
• T-ratios may be larger than actual
• More common with cross sectional data

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Regression Model For ND Points Scored
ND Points 38.54 0.079b1 - 0.170b2 -
0.662b3 - 3.16b4
b1 Total Yards Gained
b3 Total Plays
b2 Penalty Yards
b4 Turnovers
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4 Checks of a Regression Model
1. Do the coefficients have the correct sign?
2. Are the slope terms statistically significant?
3. How well does the model fit the data?
4. Is there any serial correlation?
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4 Checks of a Regression Model
1. Do the coefficients have the correct sign?
Could this represent a big play factor?
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4 Checks of a Regression Model
2. Are the slope terms statistically significant?
3. How well does the model fit the data?
4. Is there any serial correlation?
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4 Checks of a Regression Model
3. How well does the model fit the data?
Adjusted R2 74.22
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4 Checks of a Regression Model
4. Is there any serial correlation?
Data is cross sectional
With limited data points, how useful is this
regression in describing how well the model fits
the actual data? Is there a way to tests its
reliability?
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How to test the significance of the analysis
What happens when the sample size is not large
enough (n 30)?
Bootstrapping is a method for estimating the
sampling distribution of an estimator by
resampling with replacement from the original
sample.

• Commonly used statistical significance tests are
  used to determine the likelihood of a result
  given a random sample and a sample size of n.
• If the population is not random and does not
  allow a large enough sample to be drawn, the
  central limit theorem would not hold true
• Thus, the statistical significance of the data
  would not hold
• Bootstrapping uses replication of the original
  data to simulate a larger population, thus
  allowing many samples to be drawn and statistical
  tests to be calculated

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How It Works
Bootstrapping is a method for estimating the
sampling distribution of an estimator by
resampling with replacement from the original
sample.

• The bootstrap procedure is a means of estimating
  the statistical accuracy . . . from the data in a
  single sample.
• Bootstrapping is used to mimic the process of
  selecting many samples when the population is too
  small to do otherwise
• The samples are generated from the data in the
  original sample by copying it many number of
  times (Monte Carlo Simulation)
• Samples can then selected at random and
  descriptive statistics calculated or regressions
  run for each sample
• The results generated from the bootstrap samples
  can be treated as if it they were the result of
  actual sampling from the original population

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Characteristics of Bootstrapping
Full Sample
Sampling with Replacement
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Bootstrapping Example
Random sampling with replacement can be employed
to create multiple independent samples for
analysis
Limited number of observations
1st Random Sample
Original Data Set
109 Copies of each observation
Creating a much larger sample with which to work
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When it should be used
Bootstrapping is especially useful in situations
when no analytic formula for the sampling
distribution is available.

• Traditional forecasting methods, like exponential
  smoothing, work well when demand is constant
  patterns easily recognized by software
• In contrast, when demand is irregular, patterns
  may be difficult to recognize.
• Therefore, when faced with irregular demand,
  bootstrapping may be used to provide more
  accurate forecasts, making some important
  assumptions

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Assumptions and Methodology

• Bootstrapping makes no assumption regarding the
  population
• No normality of error terms
• No equal variance
• Allows for accurate forecasts of intermittent
  demand
• If the sample is a good approximation of the
  population, the sampling distribution may be
  estimated by generating a large number of new
  samples
• For small data sets, taking a small
  representative sample of the data and replicating
  it will yield superior results

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Applications and Uses
Criminology

• Statistical significance testing is important in
  criminology and criminal justice
• Six of the most popular journals in criminology
  and criminal justice are dominated by
  quantitative methods that rely on statistical
  significance testing
• However, it poses two potential problems
  tautology and violations of assumptions

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Applications and Uses
Criminology

• Tautology the null hypothesis is always false
  because virtually all null hypothesis may be
  rejected at some sample size
• Violation of assumptions of regression errors
  are homogeneous and errors of independent
  variables are normally distributed
• Bootstrapping provides a user-friendly
  alternative to cross-validation and jackknife to
  augment statistical significance testing

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Applications and Uses
Actuarial Practice

• Process of developing an actuarial model begins
  with the creation of probability distributions of
  input variables
• Input variables are generally asset-side
  generated cash flows (financial) or cash flows
  generated from the liabilities side
  (underwriting)
• Traditional actuarial methodologies are rooted in
  parametric approaches, which fit prescribed
  distribution of losses to the data

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Applications and Uses
Actuarial Practice

• However, experience from the last two decades has
  shown greater interdependence of loss variables
  with asset variables
• Increased complexity has been accompanied by
  increased competitive pressures and more frequent
  insolvencies
• There is a need to use nonparametric methods in
  modeling loss distributions
• Bootstrap standard errors and confidence
  intervals are used to derive the distribution

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Applications and Uses
Classifications Used by Ecologists

• Ecologists often use cluster analysis as a tool
  in the classification and mapping of entities
  such as communities or landscapes
• However, the researcher has to choose an adequate
  group partition level and in addition, cluster
  analysis techniques will always reveal groups
• Use bootstrap to test statistically for fuzziness
  of the partitions in cluster analysis
• Partitions found in bootstrap samples are
  compared to the observed partition by the
  similarity of the sampling units that form the
  groups.

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Applications and Uses
Human Nutrition

• Inverse regression used to estimate vitamin B-6
  requirement of young women
• Standard statistical methods were used to
  estimate the mean vitamin B-6 requirement
• Used bootstrap procedure as a further check for
  the mean vitamin B-6 requirement by looking at
  the standard error estimates and confidence
  intervals

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Application and Uses
Outsourcing

• Agilent Technologies determined it was time to
  transfer manufacturing of its 3070 in-circuit
  test systems from Colorado to Singapore
• Major concern was the change in environmental
  test conditions (dry vs humid)
• Because Agilent tests to tighter factory limits
  (guard banding), they needed to adjust the
  guard band for Singapore
• Bootstrap was used to determine the appropriate
  guard band for Singapore facility

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An Alternative to the bootstrap
Jackknife

• A statistical method for estimating and removing
  bias and for deriving robust estimates of
  standard errors and confidence intervals
• Created by systematically dropping out subsets of
  data one at a time and assessing the resulting
  variation

Bias A statistical sampling or testing error
caused by systematically favoring some outcomes
over others
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A comparison of the Bootstrap Jackknife

• Bootstrap
• Yields slightly different results when repeated
  on the same data (when estimating the standard
  error)
• Not bound to theoretical distributions

• Jackknife
• Less general technique
• Explores sample variation differently
• Yields the same result each time
• Similar data requirements

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Cross-Validation
Another alternative method

• The practice of partitioning data into a sample
  of data into sub-samples such that the initial
  analysis is conducted on a single sub-sample
  (training data), while further sub-samples (test
  or validation data) are retained blind in order
  for subsequent use in confirming and validating
  the initial analysis

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Bootstrap vs. Cross-Validation

• Bootstrap
• Requires a small of data
• More complex technique time consuming

• Cross-Validation
• Not a resampling technique
• Requires large amounts of data
• Extremely useful in data mining and artificial
  intelligence

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Methodology for ND Points Model

• Use bootstrapping on ND points scored regression
  model
• Goal determine the reliability of the model
• Replication, random sampling, and numerous
  independent regression
• Calculation of a confidence interval for adjusted
  R2

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R2 Data
Bootstrapping Results
The Mean, Standard Dev., 95 and 99 confidence
intervals are then calculated in excel from the
24 observations
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Bootstrapping Results
R2 Data

• Mean 0.8046
• STDEV 0.1131
• Conf 95 0.0453 or 75.93 - 84.98
• Conf 99 0.0595 or 74.51 - 86.41

So what does this mean for the results of the
regression?
Can we rely on this model to help predict the
number of points per game that will be scored by
the 2006 team?
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Questions?