NATCOR Regression Modelling for Time Series

Transcription

1 Universität Hamburg Institut für Wirtschaftsinformatik Prof. Dr. D.B. Preßmar Professor Robert Fildes NATCOR Regression Modelling for Time Series The material presented has been developed with the substantial help of Dr. Sven Crone

2 Forecasting Methods RECAP: Extrapolative vs. Explanatory (Causal) Methods Volume Volume Time Use only past historical data to recognise patterns and extrapolate them into the future. Promotion BOGOF Price Relationships between dependent and independent Variables are identified and assumed to hold in the future Use additional explanatory variables to predict the future. There is no a-priori superiority of either approach depends on problem & data

3 Using regression in time series forecasting research In market modelling Diffusion of new technologies Optimization of the marketing mix promotions One important issue: Decoupling Revenue management the optimization from the forecasting is suboptimal In retail, hotels, transport all models depend on demand Best models policies under certainty with sensitivity testing are often far from best under forecast uncertainty In operations Collaboration In forecasting and econometric research Aggregation, non-linearity, conditions where causal methods add value Analysis of behavioural and field experiments

4 Agenda Regression. Recap: Multiple Regression Concept of Causal Models Choosing the variables 2. Recap: Specifying Regression Models Estimating a multivariate Regression Model Validating a Regression Model 3. Extending the basic Regression for Time Series Modelling (deterministic) Seasonality Modelling Outliers & Level Shifts Including Dummy Variables for Promotions / Events Including Time Lags 4. The Model building Process

5 CAUSAL FORECASTING a simple regression model Inputs or Explanatory variables X, X 2 Output or Dependent variable, Y Y = α + β X + β X t t 2 2 t error t Past relationships in the data are identified and assumed to hold in the future Estimated model: Y = a + b X + b X t t 2 2 t error t

6 Deciding the Causal Variables Variable to be forecast Retail Petrol price How do you decide the variables? Prior Research Experience Experts Brainstorming

7 Preliminary Model Development Graphical Analysis and Preliminary Model Development Suppose we are interested in the level of gas prices as a function of various explanatory variables. Observe Gas Prices (=Y t ) over n time periods, t =, 2,, n Step : DDD (Draw the D- Diagram), a time plot of Y against time Step 2: produce a scatter plot of Y against each explanatory variable X j o For step 2, identify possible variables: E.g. Personal Disposable Income Unemployment S&P 500 Index Price of crude oil Data: Jan 996 Dec 200

8 A Case Study on the Price of Gasoline Suppose we are interested in predicting the price of (unleaded regular) gasoline (at the pump), given the price of crude oil at the refinery. We examine monthly data. The price of crude oil takes some time to have its effect on the pump price, so we lag the price of crude by one month. Define the variables: Y = Unleaded X = L_crude Q: Why else might we use a lagged value for the X variable? 8

9 Graphical Analysis and Preliminary Model Development Data: Jan 996 Dec 200

10 Regression Correlation Correlation does not imply causation!!!. The solution to fire fighting! The more firemen fighting a fire, the more damage there is going to be. Therefore firemen cause damage. 2. Reducing crime levels is simple... Since the 950s, both the atmospheric CO 2 level has increased sharply at the same time as global temperature. Hotter weather always leads time higher crime levels. Hence, reducing atmospheric CO 2 will lead to decreasing crime. But check out Bernard Shaw in Ord and Fildes

11 Graphical Analysis and Preliminary Model Development First row: Pearson correlation Second row: P-Value Measures the linear relationship Data: Jan 96-Dec 200

12 Agenda Regression. Recap: Multiple Regression Concept of Causal Models Choosing the variables 2. Recap: Specifying Regression Models Estimating a multivariate Regression Model Validating a Regression Model 3. Extending the basic Regression for Time Series Modelling (deterministic) Seasonality Modelling Outliers & Level Shifts Including Dummy Variables for Promotions / Events Including Time Lags 4. The Model building Process (Lecture 5)

13 The Multiple Regression Model Assume a linear relation between Y and X,,X p where: Y = β + β X + β X + + β X + ε p p β 0 = intercept (value of Y when all X j = 0) β j = expected effect of X j on Y, all other factors fixed ε = random error Expected value of Y given the {X j }: So Or ( X) = = β0 + β + β βp p EY Y X X X Y = [Expected value] + [Random error] Observed= Signal + Noise

14 The Method of Ordinary Least Squares (OLS) Define error = Observed Fitted e= Y EY ( X) i i i Estimate the intercept and slope coefficients by minimizing the sum of squared errors (SSE). That is, choose the coefficients { b to minimize: 0, b,, b K } n n 2 2 i ( i 0 i 2 2i K Ki) i= i= SSE = e = Y b b X b X b X

15 Least Squares Estimators Equivalent to Maximum Likelihood Unbiased On average the estimators are centred on the true parameter value Efficient E( β ˆ) = β There is no other linear estimator with a smaller variance var( ˆ) β Consistency var( β*) for any other estimator β * As the sample size, n, increases (to ) the estimator β ˆn β

16 Regression Multivariate Models Ordinary Least Squares Linear Regression (OLS) The maths = t t y y y y 2 2 ( ) p p t p t tp x x x x X x x + = = t t ε ε ε ε 2 0 ( ) p p β β β β + = y = β 0 + x β + x 2 β 2 + x 3 β 3 +. ε y 2 = β 0 + x 2 β + x 22 β 2 + x 23 β 3 +. ε 2 y t = β 0 + x t β + x t2 β 2 + x t3 β 3 +.ε t

17 Ordinary Least Squares Linear Regression (OLS) The maths y = Xβ + ε Minimize, a scalar ee Regression Multivariate Models ˆβ = ( ) X X X y TIP! Where is the constant? = p x 2 p if X β t p 0 x x tp is a constant X = transpose of matrix X, switch between rows and columns X - = inverse of matrix X, XX - = I

18 Testing Individual Coefficients Y = β + β X + β X + + β X + ε p p Test of a coefficient H 0 : slope coefficient for X i is zero [X i does not add value to model, given other variables in the model] H A : slope coefficient for X i is not zero [X i adds value, given other variables in the model] Test Statistic t = b i / SE(b i ) b i = slope coefficient SE(b i ) = standard error of b i

19 Testing the Model The estimated coefficients are random unbiased The decision rule for testing the individual model coefficients is: o Reject H 0 if P < α o where P is the observed significance level, and o α is the significance level used for testing, typically o The rule implies that we do not reject H 0 if P > α. o Rule applies for all testing including F test for model With a variable insignificant with Null hypothesis of no linear effect, i.e b =0, what do we then do? Y = β + β X + β X + + β X + ε p p

20 The Multiple Regression Model Example 8.: The Regression Model for Unleaded Gasoline prices The regression equation is Unleaded = L_ crude L_Unemp L_SP L_PDI Predictor Coef SE Coef T P Constant L_ crude L_ Unemp L_SP L_PDI Q: Interpretation do model make sense?

21 How can we compare different models? Let s look at fit. Date Unleaded Crude ResCrude ResPDI 5-Jan Feb Yˆ i = a + 5-Mar bx 5-Apr i 5-May e i = Y i 5-Jun Yˆ residual 5-Jul i = 5-Aug Sep Oct Nov Dec RMSE What are good measures of the fit of our model. the individual residuals tell us everything, but can we summarise their information?

22 Partition of the Sum of Squares Total Sum of Squares: How Good is the Fitted Line? SST = S = ( Y Y ) Sum of Squared Errors (Unexplained Variation): Sum of Squares accounted for by the regression equation: i= n The sums of squares are partitioned: SST = SSR + SSE Our aim: to build a model that explains the variation in Y, SST i= n i= YY i= n i= SSE = ( Y Yˆ ) SSR = ( Yˆ Y ) i= i i i 2 2 i 2

23 ERROR VARIANCE The residual variation is also used to calculate the error variance (and standard deviation) 2 ( ) 2 Y 2 i Yi s = σ = n p It measures the accuracy with which the model explains the data. The accuracy of a prediction using the model depends on it. MINITAB & SPSS carries out the calculation automatically calculating a confidence interval, I. ie. 90% of the time, actual lies within the Range. [ Υˆ Ι, Υˆ Ι] You need to select the 'confidence' probability within which you expect your the observation to lie. p p +

24 Explained MS F = = Un explained MS = n i= n i= ( Yˆ Y) i ( Y Yˆ ) i i 2 2 K ( n K ) F-test Explained SS Explained df Unexplained SS Unexplained df MS = Mean Square SS = Sum of Squares df = degree of freedom m = number of parameters (coefficients in equation) = K+ with K explanatory variables and a constant n = number of observations

25 SUMMARY STATISTICS The 'F' statistic and associated p value measures the overall explanatory power of the explanatory (independent) variables. The model should be chosen so that at least some elements of it are likely to have strong explanatory power. The F test is therefore not particularly important. The 't' tests measure the impact of individual variables. Methods of comparing alternative models are therefore needed. Commonly used * R 2 - a measure of the overall adequacy of the model * σ or s - the standard deviation of the error term - measures the uncertainty around the predictions i.e. the model fit

26 TESTING THE ADEQUACY OF A MODEL Is this a good model Criteria include * Interpretation of the estimated model * Strength of relationship between Xs and Y * Overall adequacy of Model * Validity of Assumptions * Errors in Predictions No simple rules

27 Linear model is: STRENGTH OF THE RELATIONSHIP Does the independent variable (x) affect the dependent variable (y) Y = β0 + β X + ( error) A unit change in X produces an (linear additive) impact of β in Y. Interpreting the Model: should make sense, i.e correct sign, low standard error Elasticity The proportionate change in Y relative to the proportionate change in X is measured as: Y Y X X

28 Assumption of normality Checking the Assumptions Plot a histogram Plot a P-P plot Statistical tests:. Kolmogorov-Smirnov 2. Chi-square

29 Assumption of constant variance Checking the Assumptions Most of the residuals contained in a fixed corridor constant variance Statistical tests:. Split residuals in two sub-samples 2. Hypothesis testing: a. H0: σ = σ 2 b. Hα: σ σ 2 Also check for:. Systematic patterns 2. Trend 3. Seasonality None should be present!

30 Unpredictability of residuals Checking the Assumptions Plot residuals against:. Predicted values (Ŷ) 2. All explanatory variables (X) 3. Time There should be no patterns Linear Nonlinear patterns What is this point? Outlier? Statistical tests:. Correlation Looks more or less random

31 Detecting serial correlation on residuals Residual Autocorrelation Function (ACF) Checking the Assumptions Sample Autocorrelation Strong serial correlation Model invalid! χ Lag Statistical tests: Ljung- Box test Residuals should be uncorrelated There should be no significant lags Can be used to check for season/trend (there should be none!) 2 LB = r where r is the k th order autocorrelation 2 k rk = corr(e t,e t k) k coefficient With degrees of freedom

32 Checking the Assumptions

33 Analysis of Residuals for Gas Price Data Residuals appear to be approximately normal (Probability Plot and Histogram), but there are some outliers o Check the original data to identify the outliers and to determine possible explanations Model does not capture time dependence Zig-zag pattern in Residuals vs. Order Errors are not homoscedastic See in Residuals vs. Fitted Value Increased volatility in the later part of the series See in Residuals vs. Order Some evidence of seasonal pattern Look for peaks every 2 months in Residuals vs. Order

34 Analysis of Residuals for Gas Price Data Graph residuals The Autocorrelation Function (ACF) allows us to check for dependence at a range of possible lags. Ljung-Box stat rk = corr(e t,e t k) 2 LB = r k NB: The Durbin-Watson test examines only first order autocorrelation: invalid with lags Values centred at between 0 and 4 2

35 Forecasting with Multiple Regression Example: One-step-ahead forecasts for gas prices We use the four-variable model for gas prices as an illustration. Crude, Unemployment, S&P, PDI Estimate the model to December 2008 One-step-ahead forecasts were generated from equation, the forecast for January 2009 uses the December 2008 values of the explanatory variables. The regression model is Unleaded = L_crude L_Unemp L_S&P L_PDI The values for December 2008 are as follows: L_crude: 4.2, L_Unemp: 7.3, L_S&P: 888.6, L_PDI: The forecast for January 2009 is then F = * * * * =.889.

36 Forecasting with Multiple Regression Given values of the inputs The point forecast is given by: F = b + bx + bx + + b X The Prediction Interval is given by: F ± t ( n K ) * SE( Y F ). where t denotes the appropriate percentage point from t- tables Assumes Xs are known Example { X, X,, X } n+, n+,2 n+, K n+ 0 n+, 2 n+,2 K n+, K n+ α /2 n+ n+ K = 4 and n = 55, so DF = 50. The SE for the point forecast is found to be (use R, not SPSS). Using t (50) =.976, we find that the 95 percent prediction interval is.8975 ± (.976)*(0.695) =.8975 ± = [.563, 2.233].

37 Issues in Multivariate Regression Establishing the variables to include Testing for insignificant coefficients (& variables) Estimation, summary statistics and testing (same as in the bivariate single explanatory variable case) Simplifying the model Removing inappropriate variables Avoiding spurious relationships Comparing different models

38 Summary - A Multivariate Example The process of model building Establish a default model Graph the data Any features that need explaining? Estimate models Check assumptions: Diagnostic checks Compare models Interpretation Summary statistics (is the model fit for purpose?) Revise model

39 Gas prices model building I. Graph focus variable retail price 2. Identify possible explanatory variables L_Personal Disposable Income L_Unemployment L_S&P 500 Index L_Price of crude oil Seasonal dummies And? 3. Estimated model is: + seasonal dummies Model Coefficients a U sta da d ed Coefficients B Std. Error t Sig. L_Crude L_PDI 4.994E L_Unemp L_Prod L_SP 4.078E

40 Gas prices model building II Summary Stats Diagnostics Outliers? Residuals (approx normal), homoscedastic Serial (auto) correlation? Tests, graphs Add lags Relationship with Xs? Stability Split? Forecasting performance Compared to other regression specifications + univariate benchmark

41 Take-Aways Start the modeling process by careful consideration of available theory and previous empirical studies Carry out a full preliminary analysis of the data to look for associations and for unusual observations Test both the overall model and the individual components Examine the validity of the underlying assumptions Make sure that the model is sensible with respect to the signs and magnitudes of the slope coefficients Use a hold-out sample to evaluate forecasting performance.

42 Questions? Basic reference: Ord, K and Fildes, R (203) Principles of Business Forecasting, South- Western, Cengage