1 babs 502 lecture 1 february 23, 2009 (c) martin l. puterman
Post on 18-Dec-2015
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TRANSCRIPT
2
Bookkeeping
• Your instructor• Course guidelines
– Lectures
– Assignments
– Project – no exam
– Contest
– Software –NCSS
• Case Study
(C) Martin L. Puterman
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What is a Forecast?A prediction of the future
fore = before + cast = throwLiterally planning before you throw.
There is some confusion about this pointOften organizations refer to direct outputs of decisions as forecasts. (Sometimes it is easier to use this terminology)
Example – “production forecasts” are not “forecasts”They are subject to variability but are known to some
degree of accuracy by organization members.
(C) Martin L. Puterman
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Course Themes
• Forecasts are necessary for effective decision making– Forecasting, planning and control are interrelated
• Forecasts are usually (almost always) wrong– Quantifying forecast variability is as important as
determining the forecast; it is the basis for decision making.
– Rare events happen and can have significant impact on forecasts
• Scientific methods improve forecasting
(C) Martin L. Puterman
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Course Objectives
• To provide a structured and objective approach to forecasting
• To provide hands on experience with several popular forecasting methods
• To determine the data requirements for effective forecasting
• To integrate forecasting with management decision making and planning
• To introduce you to some advanced forecasting methods
(C) Martin L. Puterman
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Why Forecast?
• It’s fun• To look smart• But most importantly: To make better decisions
– Investments– Inventory– Staff – Medical treatment timing
• Fact: Forecasts are usually (always?) wrong! – Why do it then? – Because you have to!!
• Effect of bad forecasts– Excess costs – too much staff or stock– Poor service –waiting lines and stockouts
(C) Martin L. Puterman
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Knowledge Base for Effective Forecasting
• Subject Matter Knowledge– Industry
– Market
– Demand Sources
• Statistics
• Statistical software and IT
• Interpersonal skills– acquiring data
– report writing
– presentations
– team work
(C) Martin L. Puterman
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Forecasting Applications
• Demand forecasts – Whistler-Blackcomb - staffing– TELUS – capacity expansion – Worksafe BC – staffing, budgeting and reserve planning– Health Authorities – staffing, scheduling and planning – Mike’s Products - production and inventory decisions
• Price forecasts– Teck- Cominco - production planning, ore purchase– Vancouver Olympic Village – resale value
• New market forecasts; – Webvan, Petfood.com, Napster
• Technology forecasts– Intel; Nortel; TELUS; Microsoft; Google
(C) Martin L. Puterman
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The Challenge
• Enhance the performance of the inventory and
distribution system for products in the US market
• Highly competitive market with highly seasonal
demand patterns
• Client’s Goal - Get the right product in the right
quantity to the right customer on time!
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The Production/Distribution System
Co-packers
Distribution Centers
Retailers (many)
Products
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Modeling• A linear programming based planning tool
• For each SKU it finds for the next 12 months:
- Optimal co-packer production levels
- Optimal distribution and transshipment plans
- Optimal distribution center (DC) inventory levels
• Developed for operational decisions but first used for
tactical/strategic decisions
• Implemented in Excel using Frontline Solver
• User friendly interface
(C) Martin L. Puterman
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Using the Model in PracticeMonth Date Steps to Take
T – 1 20th Provide forecasts for month T to T + 12
T 5th Estimate closing inventory at the end of month T, using
- Opening inventory of month T,
- Production schedule of month T, and
- Actual order from distributors and DC re-order suggestions in month T
Monthly input data check list, including
- Unit costs
- Production and inventory capacity
- Minimum and fixed production
From production and distribution personnel. Document the changes to the data.
6-9th - Run the tool with updated data, review the output and re-run if necessary.
- Set production plan for month T + 1
- Document changes of actual plan from tool output and reasons of changes
10th Provide co-packers with production plan for month T+1
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Forecasts drive the model!
• Key input – Forecasts by sales region by SKU for next 12 months.– Produced by regional sales representatives
– Accuracy declines over 12 month period
– Not calibrated but good in aggregate!
• But model is used in a rolling horizon approach
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Forecasting is NOT a Statistical Topic
• Primary interest is not in hypothesis tests or confidence intervals.
• Underlying models developed in statistics arena are often used: – regression– time series – neural networks – dynamic Bayesian systems and state space models
• Forecasts must be assessed on– the quality of the decisions that are produced
– their accuracy
(C) Martin L. Puterman
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Types of Forecasting• Extrapolation
– Based on previous data patterns • Assumes past patterns hold in future
– Exponential Smoothing, Trend Models, ARIMA models
• Causal – Based on factors that might influence the quantity being forecasted
• Assumes past relationships hold in the future
– Regression
• Judgemental– Based on individual knowledge– Sales force composites, expert opinion, consensus methods– Surveys and market research
• Collaborative– Based on information available to supply chain partners– Information sharing and partnerships
(C) Martin L. Puterman
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Forecasting Considerations
• Forecasts vs. Targets• Short Term vs. Medium Term vs. Long term• One Series vs. Many• Seasonal vs. Non-seasonal• Simple vs. Advanced• One-Step Ahead vs. Many Steps Ahead• Automatic vs. Manual• Exceptions• When to update models
(C) Martin L. Puterman
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Forecasting Horizons
Short term a few days or weeks
Medium term usually a few months to 1 or 2 years
Long term usually more than 2 year
Why distinguish between these? Different methods are more suitable in each case.
(C) Martin L. Puterman
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Some Forecasting Observations
He who lives by the crystal ball soon learns to eat ground glass.– Edgar R. Fiedler in The Three Rs of Economic Forecasting-Irrational, Irrelevant and
Irreverent , June 1977.
Prediction is very difficult, especially if it's about the future. – Nils Bohr, Nobel laureate in Physics – This quote serves as a warning of the importance of testing a forecasting model out-of-sample. It's
often easy to find a model that fits the past data well--perhaps too well!--but quite another matter to find a model that correctly identifies those features of the past data which will be replicated in the future
There is no reason anyone would want a computer in their home.– President, Chairman and founder of Digital Equipment Corp, 1977
640K ought to be enough for anybody.– Bill Gates, 1981
Our sales forecasts are accurate in aggregate– Many marketing directors
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Forecasting methods that work
• Naïve: Last Period or Same Period Last Year
• Regression– Extrapolation
– Causal
• Exponential Smoothing– Simple
– Trend / Damped Trend
– Holt-Winters
• Pooled methods
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Forecasting methods I don’t recommend
• Crystal balls• Tea leaves• Fortune cookies• Expert Opinion• Complex statistical models
– Box-Jenkins / ARIMA Models
– Multivariate Econometric Models
– Neural Networks
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Forecasting Practice in Organizations
• What quantities do organizations need to forecast? • What methods are users familiar with? • What methods have been used? • What are the impediments to using quantitative
techniques?• What factors which make forecasting most
difficult?
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What do organizations need to forecast?• Costs
– raw materials– semi-finished goods– wage rates and overheads – interest rates
• Sales/ Activities– by industry, by region– by market/product, market share– by product category, by wholesaler, by retailer– new product sales– competitive position - e.g. prices, exchange rates– competitive behaviour– customer service– price
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What do organizations need to forecast?
• Technology– new products– new processes– diffusion rates
• Social and Political trends– demographics– wealth profile– welfare and health provisions– impact of technology
• Projects– duration– costs– life cycle maintenance
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Top 10 impediments to effective forecasting
10. Absence of a forecasting function9. Poor data 8. Lack of software7. Lack of technical knowledge6. Poor data5. Lack of trust in forecasts4. Poor data3. Too little time2. Not viewed as important 1. Poor data
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Forecasting Challenges• Technical Issues
– What is the best approach
• Organizational Issues– reporting structures– accountability– incentive systems
• Information – historical data not available– timeliness and reliability– what information is required when
• Users – conflicting objectives
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Top Down vs. Bottom Forecasting
Top-down Bottom Up Strengths Aggregate market information included Detailed customer info Marketing plans Responsibility clear for sales Competitive viewpoint Motivation Weaknesses No responsibility accepted by sales
force Aggregated forecast may not reflect market plans
Confuses forecasts with aggregate target setting
No easy reconciliation with corporate financial projections
Politically motivated May be biased due to sales force compensation schemes
Costs – more staff time and slower process
Top Down - Forecast at central office
Bottom up - Forecast by sales force
Questions – which is more accurate? which should be used?
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Responsibilities of Units• Production
– Acquiring materials– Planning and scheduling production runs
• Logistics– Delivering products to customers
• Marketing– Generating orders– Creating product demand
• IT– Acquiring software– Integrating software– Managing data
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Scientific Forecasting
• Requires familiarity with very basic statistical concepts:– Mean, standard deviation, skewness and kurtosis
– medians and percentiles
– histograms, stem and leaf plots, box plots
– scatter plots, correlation, regression
If you’re not keeping score you are only practicing!
(C) Martin L. Puterman
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The Forecasting Process - I
• Determine what is to be forecasted and at what frequency
• Obtain data• Process the data• PLOT THE DATA• Clean the data• Hold out some data
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The Forecasting Process - II
• Obtain candidate forecasts• Assess their quality
– Forecast accuracy on hold out data– Do they make sense?– Do they produce good decisions?
• Revise forecasts• Recalibrate model on full data set• Produce forecasts and adjust as necessary• Produce report• In future - Evaluate accuracy of forecasts
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Means and Standard Deviations
Means and standard deviations are only useful for summarizing data when it looks like it comes from a normal distribution
-3 -2.50
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
3.00
They especially are not appropriate for summarizing time series data with trends or seasonality.
(C) Martin L. Puterman
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Some Normal Distribution Properties
• Determined completely by its mean and standard deviation • Its skewness is 0 and its kurtosis is 0• 95% of the observations fall within 2 standard deviations (not standard
errors!) of the mean – Useful for determining forecast ranges– Usually forecasts are accurate to 2 standard deviations
• 95% of the observations fall below + 1.645
– Useful for determining service levels of inventory policies• When extreme outliers may occur, the normal distribution may not be
appropriate– Such distributions are said to have long tails – These distributions have positive kurtosis.– The book, The Black Swan, by Nicholas Taleb addresses the practical significance of
this issue.
(C) Martin L. Puterman
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Data Patterns
Diagram 1.2: Seasonal - more or less regular movements
w ithin a year
0
20
40
60
80
100
120
Year 5 10
15
20
25
30
35
40
45
Diagram 1.1: Trend - long-term growth or decline occuring
w ithin a series
0
20
40
60
80
100
Year 3 6 9 12
15
18
21
24
27
30
Diagram 1.3: Cycle - alternating upswings of varied length
and intensity
0
2
4
6
8
10
Year 5 10
15
20
25
30
35
40
45
Diagram 1.4: Irregular - random movements and those which
reflect unusual events
0
50
100
150
200
250
300
350
1 10
19
28
37
46
55
64
73
82
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Basic Modeling ConceptAn observed measurement
is made up of a systematic part
and a random part
Unfortunately we cannot observe either of these.Forecasting methods try to isolate the systematic part.
Forecasts are based on the systematic part.The random part determines the distribution shape and
forecast accuracy.
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Basic Concept Again
Observed Value =
Signal “+” NoiseIn non-normal (or non-additive) models the “+” may
be inappropriate
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Forecasting Notation (p.71)
t a specific time periodn total number of observationsYt observed value at time tFt+k forecasted value k periods ahead at time t
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Correlation
• Measures the strength of the (linear) relationship between two measurements
• Often denoted by XY
• A number between -1 and +1
• Answers question: Does one measurement contain information about another measurement?
• Theoretically XY = Cov(X,Y)/X Y
• From a sample rXY (see equation 2.8).
(C) Martin L. Puterman
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Autocorrelation - What is it?
• Correlation between observations at different time points in a time series - estimated by rk
– Lag 1 autocorrelation measures the correlation between Yt and Yt-1
– Lag k autocorrelation measures the correlation between Yt and Yt-k
• Summarized in terms of an autocorrelation function (ACF) which give the autocorrelations between observations at all lags.– It is often represented graphically as a plot of autocorrelation vs.
lag
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Autocorrelation - Why is it useful?
• Can the past help predict the future?– if autocorrelations at all lags are near zero then best
predictor is historical mean
– if all autocorrelations of differences of series are near zero then best predictor of the future is the current value
– if autocorrelations at seasonal lags are large - suggests seasonality in data
• An important component of the ARIMA or Box-Jenkins’ method
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Autocorrelation Example 1
-1.0
00
-0.5
00
0.0
00
0.5
00
1.0
00
0 10 21 31 41
Autocorrelations of C2 (0,0,12,1,0)
Time
Auto
corr
ela
tions
-2.0
-0.8
0.5
1.8
3.0
1 17 34 50 67
Plot of C2
Time
C2
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Autocorrelation Example 24
.85
.15
.45
.76
.0
1 19 37 55 73
Plot of Wages
Time
Wages
-1.0
00
-0.5
00
0.0
00
0.5
00
1.0
00
0 10 21 31 41
Autocorrelations of Wages (0,0,12,1,0)
Time
Auto
corr
ela
tions
-1.0
00
-0.5
00
0.0
00
0.5
00
1.0
00
0 10 21 31 41
Autocorrelations of Wages (1,0,12,1,0)
Time
Auto
corr
ela
tions
Difference
Original
(C) Martin L. Puterman