Transcript lecture note 7

STAT 497
LECTURE NOTES 7
FORECASTING
1
FORECASTING
• One of the most important objectives in time
series analysis is to forecast its future values.
It is the primary objective of modeling.
• ESTIMATION (tahmin)the value of an
estimator for a parameter.
• PREDICTION (kestirim)the value of a r.v.
when we use the estimates of the parameter.
• FORECASTING (öngörü)the value of a future
r.v. that is not observed by the sample.
2
FORECASTING
Yt  Yt 1  at

ˆ (Estimate of  )

Yˆt  ˆYt 1 (Predictio n)

Yˆt 1 (Forecast)
3
FORECASTING FROM AN ARMA MODEL
THE MINIMUM MEAN SQUARED ERROR FORECASTS
Observed time series, Y1, Y2,…,Yn.
Observed sample
Y1
Y2 …………………..
Yn
Yn+1? Yn+2?
n: the forecast origin
Yˆn 1  the forecast value of Yn1
Yˆn 2   the forecast value of Yn 2
Yˆn    the forecast value of Yn
  step  ahead forecast of Yn
 minimum MSE forecast of Yn
4
FORECASTING FROM AN ARMA
MODEL
Yˆn    E Yn Yn , Yn1 ,, Y1 
 The conditional expectatio n of Yn
given the observed sample
5
FORECASTING FROM AN ARMA MODEL
• The stationary ARMA model for Yt is
 p B Yt  0   q B at
or
Yt  0  1Yt 1     pYt  p  at  1at 1     q at q
• Assume that we have data Y1, Y2, . . . , Yn and
we want to forecast Yn+l (i.e., l steps ahead
from forecast origin n). Then the actual value
is
Yn  0  1Yn1     pYn p  an  1an1     q anq
6
FORECASTING FROM AN ARMA MODEL
• Considering the Random Shock Form of the
series
q B 
Yn   0    B at   0 
at
 p B 
  0  an  1an1  2 an2     an  
7
FORECASTING FROM AN ARMA MODEL
• Taking the expectation of Yn+l , we have
Yˆn    E Yn Yn , Yn1 ,, Y1 
  an   1an1  
where
 0, j  0
E an j Yn ,, Y1   
an j , j  0
8
FORECASTING FROM AN ARMA MODEL
• The forecast error:
en    Yn   Yˆn  
 an   1an   1     1an 1
 1
  i an  i
i 0
• The expectation of the forecast error: E en    0
• So, the forecast in unbiased.
• The variance of the forecast error:
 1
 1
2
2


Var en    Var   i an i    a  i
 i 0

i 0
9
FORECASTING FROM AN ARMA MODEL
• One step-ahead (l=1)
Yn1   0  an1  1an  2 an1  
Yˆn 1   0  1an  2 an1  
en 1  Yn1  Yˆn 1  an1
Var en 1  
2
a
10
FORECASTING FROM AN ARMA MODEL
• Two step-ahead (l=2)
Yn 2   0  an 2  1an1  2 an  
Yˆn 2    0  2 an  
en 2   Yn 2  Yˆn 2   an 2  1an1
Var en 2   a2 1  12 
11
FORECASTING FROM AN ARMA MODEL
• Note that,
lim Yˆn      0
 
lim Var en     0  
 
• That’s why ARMA (or ARIMA) forecasting is
useful only for short-term forecasting.
12
PREDICTION INTERVAL FOR Yn+l
• A 95% prediction interval for Yn+l (l steps ahead)
is
Yˆn    1.96 Var en  
 1
Yˆn    1.96 a  i2
i 0
For one step-ahead the simplifies to
Yˆn 1  1.96 a
For two step-ahead the simplifies to
2
ˆ
Yn 2  1.96 a 1  1
• When computing prediction intervals from data, we substitute
estimates for parameters, giving approximate prediction intervals13
REASONS NEEDING A LONG REALIZATION
• Estimate correlation structure (i.e., the ACF and PACF)
functions and get accurate standard errors.
• Estimate seasonal pattern (need at least 4 or 5
seasonal periods).
• Approximate prediction intervals assume that
parameters are known (good approximation if
realization is large).
• Fewer estimation problems (likelihood function better
behaved).
• Possible to check forecasts by withholding recent data .
• Can check model stability by dividing data and
analyzing both sides.
14
REASONS FOR USING A PARSIMONIOUS
MODEL
• Fewer numerical problems in estimation.
• Easier to understand the model.
• With fewer parameters, forecasts less sensitive to
deviations between parameters and estimates.
• Model may applied more generally to similar
processes.
• Rapid real-time computations for control or other
action.
• Having a parsimonious model is less important if
the realization is large.
15
EXAMPLES
• AR(1)
• MA(1)
• ARMA(1,1)
16
UPDATING THE FORECASTS
• Let’s say we have n observations at time t=n
and find a good model for this series and
obtain the forecast for Yn+1, Yn+2 and so on. At
t=n+1, we observe the value of Yn+1. Now, we
want to update our forecasts using the original
value of Yn+1 and the forecasted value of it.
17
UPDATING THE FORECASTS
The forecast error is
 1
en    Yn  Yˆn     i ani
i 0
We can also write this as
en1   1  Yn11  Yˆn1   1


i 0
i 0
  i an11i   i ani
 1
  i ani   an
i
0

en   
18
UPDATING THE FORECASTS
Yn  Yˆn 1   1  Yn  Yˆn     an
Yˆn    Yˆn1   1   an
Yˆn    Yˆn1   1   Yn  Yˆn 1 1
Yˆn1    Yˆn   1   Yn1  Yˆn 1
n=100
Yˆ1011  Yˆ100 2  1Y101  Yˆ100 1
19
FORECASTS OF THE TRANSFORMED
SERIES
• If you use variance stabilizing transformation,
after the forecasting, you have to convert the
forecasts for the original series.
• If you use log-transformation, you have to
consider the fact that
EYn Y1,,Yn   expElnYn lnY1,,lnYn 
20
FORECASTS OF THE TRANSFORMED
SERIES
• If X has a normal distribution with mean 
and variance 2,
2


.
Eexp X   exp  


2


• Hence, the minimum mean square error
forecast for the original series is given by
1
ˆ

expZ n    Varen   where Zn    lnYn   
2


  EZn Z1 ,, Zn 
 2  Var Z n   Z1 , , Z n 
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MEASURING THE FORECAST
ACCURACY
22
MEASURING THE FORECAST
ACCURACY
23
MEASURING THE FORECAST
ACCURACY
24
MOVING AVERAGE AND EXPONENTIAL
SMOOTHING
• This is a forecasting procedure based on a
simple updating equations to calculate
forecasts using the underlying pattern of the
series. Not based on ARIMA approach.
• Recent observations are expected to have
more power in forecasting values so a model
can be constructed that places more weight
on recent observations than older
observations.
25
MOVING AVERAGE AND EXPONENTIAL
SMOOTHING
• Smoothed curve (eliminate up-and-down
movement)
• Trend
• Seasonality
26
SIMPLE MOVING AVERAGES
• 3 periods moving averages
Yt = (Yt-1 + Yt-2 + Yt-3)/3
• Also, 5 periods MA can be considered.
Period
Actual
3 Quarter MA Forecast
5 Quarter MA forecast
Mar-83
239.3
Missing
Missing
Jun-83
239.8
Missing
Missing
Sep-83
236.1
Missing
Missing
Dec-83
232
238.40
Missing
Mar-84
224.75
235.97
Missing
Jun-84
237.45
230.95
234.39
Sep-84
245.4
231.40
234.02
Dec-84
251.58
235.87
235.14
…
So on..
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SIMPLE MOVING AVERAGES
• One can impose weights and use weighted
moving averages (WMA).
Eg Y t = 0.6Yt-1 + 0.3Yt-2+ 0.1Yt-2
• How many periods to use is a question; more
significant smoothing-out effect with longer lags.
• Peaks and troughs (bottoms) are not predicted.
• Events are being averaged out.
• Since any moving average is serially correlated,
any sequence of random numbers could appear
to exhibit cyclical fluctuation.
28
SIMPLE MOVING AVERAGES
• Exchange Rates: Forecasts using the SMA(3)
model
Date
Rate
Three-Quarter
Moving Average
Mar-85
Jun-85
Se-85
Dec-85
Mar-86
257.53
250.81
238.38
207.18
187.81
missing
missing
248.90
232.12
211.12
Three-Quarter
Forecast
missing
missing
missing
248.90
232.12
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SIMPLE EXPONENTIAL SMOOTHING
(SES)
• Suppressing short-run fluctuation by
smoothing the series
• Weighted averages of all previous values with
more weights on recent values
• No trend, No seasonality
30
SIMPLE EXPONENTIAL SMOOTHING
(SES)
• Observed time series
Y1, Y2, …, Yn
• The equation for the model is
St  Yt 1  1   St 1
where : the smoothing parameter, 0    1
Yt: the value of the observation at time t
St: the value of the smoothed obs. at time t.
31
SIMPLE EXPONENTIAL SMOOTHING
(SES)
• The equation can also be written as
St  St 1   Yt 1  St 1 

the forecast error
• Then, the forecast is
St 1  Yt  1   St
 St   Yt  St 
32
SIMPLE EXPONENTIAL SMOOTHING
(SES)
• Why Exponential?: For the observed time series
Y1,Y2,…,Yn, Yn+1 can be expressed as a weighted
sum of previous observations.
Yˆt 1  c0Yt  c1Yt 1  c2Yt 2  
where ci’s are the weights.
• Giving more weights to the recent observations,
we can use the geometric weights (decreasing
by a constant ratio for every unit increase in
lag).
i
 ci   1    ; i  0,1,...;0    1.
33
SIMPLE EXPONENTIAL SMOOTHING
(SES)
• Then,
0
1
2
ˆ
Yt 1   1    Yt   1    Yt 1   1    Yt 2  
Yˆt 1  Yt  1   Yˆt 1 1
St+1
St
34
SIMPLE EXPONENTIAL SMOOTHING
(SES)
• Remarks on  (smoothing parameter).
– Choose  between 0 and 1.
– If  = 1, it becomes a naive model; if  is close to
1, more weights are put on recent values. The
model fully utilizes forecast errors.
– If  is close to 0, distant values are given weights
comparable to recent values. Choose  close to 0
when there are big random variations in the data.
–  is often selected as to minimize the MSE.
35
SIMPLE EXPONENTIAL SMOOTHING
(SES)
• Remarks on  (smoothing parameter).
– In empirical works, 0.05    0.3 commonly used.
Values close to 1 are used rarely.
– Numerical Minimization Process:
•
•
•
•
Take different  values ranging between 0 and 1.
Calculate 1-step-ahead forecast errors for each .
Calculate MSE for each case.
Choose  which has the min MSE.
n
et  Yt  St  min  e  
t 1
2
t
36
SIMPLE EXPONENTIAL SMOOTHING
(SES)
• EXAMPLE:
Time
Yt
St+1 (=0.10)
(YtSt)2
1
5
-
-
2
7
(0.1)5+(0.9)5=5
4
3
6
(0.1)7+(0.9)5=5.2
0.64
4
3
(0.1)6+(0.9)5.2=5.08
5.1984
5
4
(0.1)3+(0.9)5.28=5.052
1.107
TOTAL
10.945
SSE
MSE 
 2.74
n 1
• Calculate this for =0.2, 0.3,…,0.9, 1 and compare
the MSEs. Choose  with minimum MSE
37
SIMPLE EXPONENTIAL SMOOTHING
(SES)
• Some softwares automatically chooses the
optimal  using the search method or nonlinear optimization techniques.
INITIAL VALUE PROBLEM
1. Setting S1 to Y1 is one method of initialization.
2. Take the average of, say first 4 or 5
observations and use this as an initial value.
38
DOUBLE EXPONENTIAL SMOOTHING
OR HOLT’S EXPONENTIAL SMOOTHING
• Introduce a Trend factor to the simple
exponential smoothing method
• Trend, but still no seasonality
SES + Trend = DES
• Two equations are needed now to handle the
trend.
St  Yt 1  1   St 1  Tt 1 ,0    1
Tt   St  St 1   1   Tt 1 ,0    1
Trend term is the expected increase or decrease per unit time
39
period in the current level (mean level)
HOLT’S EXPONENTIAL SMOOTHING
• Two parameters :
 = smoothing parameter
 = trend coefficient
• h-step ahead forecast at time t is
Yˆt h   St  hTt
Current level
Current slope
• Trend prediction is added in the h-step ahead
forecast.
40
HOLT’S EXPONENTIAL SMOOTHING
• Now, we have two updated equations. The
first smoothing equation adjusts St directly for
the trend of the previous period Tt-1 by adding
it to the last smoothed value St-1. This helps to
bring St to the appropriate base of the current
value. The second smoothing equation
updates the trend which is expressed as the
difference between last two values.
41
HOLT’S EXPONENTIAL SMOOTHING
• Initial value problem:
– S1 is set to Y1
– T1=Y2Y1 or (YnY1)/(n1)
 and  can be chosen as
the value between 0.02< ,<0.2
or by minimizing the MSE as in SES.
42
HOLT’S EXPONENTIAL SMOOTHING
• Example: (use  = 0.6, =0.7; S1 = 4, T1= 1)
Holt
Holt
time
Yt
St
Tt
1
3
4
1
2
5
3.8
0.64
3
4
4.78
0.74
4
-
4.78+0.74
5
-
4.78+2*0.74
43
HOLT-WINTER’S EXPONENTIAL
SMOOTHING
• Introduce both Trend and Seasonality factors
• Seasonality can be added additively or
multiplicatively.
• Model (multiplicative):
Yt 1
St  
 1   St 1  Tt 1 
It s
Tt   St  St 1   1   Tt 1
Yt
I t    1   I t  s
St
44
HOLT-WINTER’S EXPONENTIAL
SMOOTHING
Here, (Yt /St) captures seasonal effects.
s = # of periods in the seasonal cycles
(s = 4, for quarterly data)
Three parameters :
 = smoothing parameter
 = trend coefficient
 = seasonality coefficient
45
HOLT-WINTER’S EXPONENTIAL
SMOOTHING
• h-step ahead forecast
Yˆt h   St  hTt I t h s
• Seasonal factor is multiplied in the h-step
ahead forecast
 , and  can be chosen as
the value between 0.02< ,,<0.2
or by minimizing the MSE as in SES.
46
HOLT-WINTER’S EXPONENTIAL
SMOOTHING
• To initialize Holt-Winter, we need at least one
complete season’s data to determine the
initial estimates of It-s.
• Initial value:
s
1.S0   Yt / s
t 1
1  Ys 1  Y1 Ys  2  Y2
Ys  s  Ys 
2.T0  

 

s s
s
s

s
2s




or T0    Yt / s     Yt / s  / s
t 1
 t  s 1 
47
HOLT-WINTER’S EXPONENTIAL
SMOOTHING
• For the seasonal index, say we have 6 years
and 4 quarter (s=4).
STEPS TO FOLLOW
STEP 1: Compute the averages of each of 6
years.
4
An   Yi / 4, n  1,2, ,6  The yearly averages
i 1
48
HOLT-WINTER’S EXPONENTIAL
SMOOTHING
• STEP 2: Divide the observations by the
appropriate yearly mean.
Year
1
2
3
4
5
6
Q1
Y1/A1
Y5/A2
Y9/A3
Y13/A4
Y17/A5
Y21/A6
Q2
Y2/A1
Y6/A2
Y10/A3
Y14/A4
Y18/A5
Y22/A6
Q3
Y3/A1
Y7/A2
Y11/A3
Y15/A4
Y19/A5
Y23/A6
Q4
Y4/A1
Y8/A2
Y12/A3
Y16/A4
Y20/A5
Y24/A6
49
HOLT-WINTER’S EXPONENTIAL
SMOOTHING
• STEP 3: The seasonal indices are formed by
computing the average of each row such that
 Y1 Y5 Y9 Y13 Y17 Y21 
I1      
 /6
 A1 A2 A3 A4 A5 A6 
 Y2 Y6 Y10 Y14 Y18 Y22 
I2    


 /6
 A1 A2 A3 A4 A5 A6 
 Y3 Y7 Y11 Y15 Y19 Y23 
I3     

 /6
 A1 A2 A3 A4 A5 A6 
Y Y Y
Y
Y
Y 
I 4   4  8  12  16  20  24  / 6
 A1 A2 A3 A4 A5 A6 
50
HOLT-WINTER’S EXPONENTIAL
SMOOTHING
• Note that, if a computer program selects 0 for  and , this
does not mean that there is no trend or seasonality.
• For Simple Exponential Smoothing, a level weight near zero
implies that simple differencing of the time series may be
appropriate.
• For Holt Exponential Smoothing, a level weight near zero
implies that the smoothed trend is constant and that an
ARIMA model with deterministic trend may be a more
appropriate model.
• For Winters Method and Seasonal Exponential Smoothing,
a seasonal weight near one implies that a nonseasonal
model may be more appropriate and a seasonal weight
near zero implies that deterministic seasonal factors may
be present.
51
EXAMPLE
> HoltWinters(beer)
Holt-Winters exponential smoothing with trend and additive seasonal component.
Call:
HoltWinters(x = beer)
Smoothing parameters:
alpha: 0.1884622
beta : 0.3068298
gamma: 0.4820179
Coefficients:
[,1]
a
50.4105781
b
0.1134935
s1 -2.2048105
s2
4.3814869
s3
2.1977679
s4 -6.5090499
s5 -1.2416780
s6
4.5036243
s7
2.3271515
s8 -5.6818213
s9 -2.8012536
s10 5.2038114
s11 3.3874876
s12 -5.6261644
52
EXAMPLE (Contd.)
> beer.hw<-HoltWinters(beer)
> predict(beer.hw,n.ahead=12)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
1963
1964 49.73637 55.59516 53.53218 45.63670 48.63077 56.74932 55.04649 46.14634
Sep
Oct
Nov
Dec
1963 48.31926 55.01905 52.94883 44.35550
53
ADDITIVE VS MULTIPLICATIVE
SEASONALITY
• Seasonal components can be additive in nature or multiplicative.
For example, during the month of December the sales for a
particular toy may increase by 1 million dollars every year. Thus,
we could add to our forecasts for every December the amount of
1 million dollars (over the respective annual average) to account
for this seasonal fluctuation. In this case, the seasonality is
additive.
• Alternatively, during the month of December the sales for a
particular toy may increase by 40%, that is, increase by a factor of
1.4. Thus, when the sales for the toy are generally weak, than the
absolute (dollar) increase in sales during December will be
relatively weak (but the percentage will be constant); if the sales
of the toy are strong, than the absolute (dollar) increase in sales
will be proportionately greater. Again, in this case the sales
increase by a certain factor, and the seasonal component is thus
multiplicative in nature (i.e., the multiplicative seasonal
component in this case would be 1.4).
54
ADDITIVE VS MULTIPLICATIVE
SEASONALITY
• In plots of the series, the distinguishing characteristic
between these two types of seasonal components is
that in the additive case, the series shows steady
seasonal fluctuations, regardless of the overall level of
the series; in the multiplicative case, the size of the
seasonal fluctuations vary, depending on the overall
level of the series.
• Additive model:
Forecastt = St + It-s
• Multiplicative model:
Forecastt = St*It-s
55
ADDITIVE VS MULTIPLICATIVE
SEASONALITY
56
Exponential Smoothing Models
1. No trend and additive
seasonal variability (1,0)
3. Multiplicative seasonal variability
with an additive trend (2,1)
2. Additive seasonal variability with
an additive trend (1,1)
4. Multiplicative seasonal variability
with a multiplicative trend (2,2)
Exponential Smoothing Models
5. Dampened trend with additive
seasonal variability (1,1)
6. Multiplicative seasonal variability
and dampened trend (2,2)
• Select the type of model to fit based on the presence of
– Trend – additive or multiplicative, dampened or not
– Seasonal variability – additive or multiplicative
OTHER METHODS
(i) Adaptive-response smoothing
.. Choose  from the data using the smoothed and
absolute forecast errors
(ii) Additive Winter’s Models
.. The seasonality equation is modified.
(iii) Gompertz Curve
.. Progression of new products
(iv) Logistics Curve
.. Progression of new products (also with a limit, L)
(v) Bass Model
59
EXPONENTIAL SMOOTING IN R
General notation: ETS(Error,Trend,Seasonal)
ExponenTial Smoothing
ETS(A,N,N): Simple exponential smoothing with additive errors
ETS(A,A,N): Holt's linear method with additive errors
ETS(A,A,A): Additive Holt-Winters' method with additive errors
60
EXPONENTIAL SMOOTING IN R
From Hyndman et al. (2008):
• Apply each of 30 methods that are appropriate to
the data. Optimize parameters and initial values
using MLE (or some other method).
• Select best method using AIC:
AIC = -2 log(Likelihood) + 2p
where p = # parameters.
• Produce forecasts using the best method.
• Obtain prediction intervals using underlying state
space model (this part is done by R automatically).
***http://robjhyndman.com/research/Rtimeseries_handout.pdf
61
EXPONENTIAL SMOOTING IN R
•
•
•
•
•
ets() function
Automatically chooses a model by default using the
AIC
Can handle any combination of trend, seasonality
and damping
Produces prediction intervals for every model
Ensures the parameters are admissible (equivalent to
invertible)
Produces an object of class ets.
***http://robjhyndman.com/research/Rtimeseries_handout.pdf
62
EXPONENTIAL SMOOTING IN R
> library(tseries)
> library(forecast)
> library(expsmooth)
R automatically finds the best model.
> fit=ets(beer)
> fit2 <- ets(beer,model="MNM",damped=FALSE)
We are defining the model as MNM
> fcast1 <- forecast(fit, h=24)
> fcast2 <- forecast(fit2, h=24)
sigma: 1.2714
AIC AICc BIC
478.1877 480.3828 500.8838
63
EXPONENTIAL SMOOTING IN R
> fit
ETS(A,Ad,A)
Call:
ets(y = beer)
Smoothing parameters:
alpha = 0.0739
beta = 0.0739
gamma = 0.213
phi = 0.9053
Initial states:
l = 38.2918
b = 0.6085
s=-5.9572 3.6056 5.1923 -2.8407
sigma: 1.2714
AIC AICc BIC
478.1877 480.3828 500.8838
64
EXPONENTIAL SMOOTING IN R
65
EXPONENTIAL SMOOTING IN R
> fit2
ETS(M,N,M)
Call:
ets(y = beer, model = "MNM", damped = FALSE)
Smoothing parameters:
alpha = 0.3689
gamma = 0.3087
Initial states:
l = 39.7259
s=0.8789 1.0928 1.108 0.9203
sigma: 0.0296
AIC AICc BIC
490.9042 491.8924 506.0349
66
EXPONENTIAL SMOOTING IN R
67
EXPONENTIAL SMOOTING IN R
• GOODNESS-OF-FIT
> accuracy(fit)
ME
RMSE
MAE
MPE
MAPE
MASE
0.1007482 1.2714088 1.0495752 0.1916268 2.2306151 0.1845166
> accuracy(fit2)
ME
RMSE
MAE
MPE
MAPE
MASE
0.2596092 1.3810629 1.1146970 0.5444713 2.3416001 0.1959651
The smaller is the better.
68
EXPONENTIAL SMOOTING IN R
> plot(forecast(fit,level=c(50,80,95)))
69
EXPONENTIAL SMOOTING IN R
> plot(forecast(fit2,level=c(50,80,95)))
70