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BABS 502

BABS 502. Regression Based Forecasting February 28, 2011. Simple and Multiple Regression. A widely used set of statistical tools that are useful for: forecasting data summary adjustment for uncontrolled factors

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BABS 502

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  1. BABS 502 Regression Based Forecasting February 28, 2011 (c) Martin L. Puterman

  2. Simple and Multiple Regression • A widely used set of statistical tools that are useful for: • forecasting • data summary • adjustment for uncontrolled factors • Basic idea is to fit an equation of the following form relating a dependent variable to one or more independent variables • y = 0 + 1x1 + 2x2 + 3x3 + … • It’s power is that by choosing the y and xi’s in different ways a wide range of different effects can be taken into account. • The theoretical model assumes that each observation is subject to an additive error which is normally distributed with mean zero and the same variance for every observation so that one observes the signal and noise components in aggregate. • In forecasting the signal part provides the point forecast and the random part provides an accuracy measure. (c) Martin L. Puterman

  3. Regression in forecasting - trend extrapolation • Fit a trend to historical data • linear Yt = a + bt • quadratic Yt = a + bt + ct2 • exponential Yt = aebt or Log (Yt) = a + bt • Assumption is that the same trend occurred throughout the past and that it will persist into future • Fit using multiple regression module in NCSS or spreadsheet regression function • Extensive regression theory available to guide use (c) Martin L. Puterman

  4. Trend Regression (c) Martin L. Puterman

  5. Trend Regression Linear Trend Quadratic Trend (c) Martin L. Puterman

  6. Dummy Variables • Dummy Variables are independent variables in regression that assume the values of either 0 or 1. • A value 1 means a condition is present; a value 0 means it is not. • When an observation in regression corresponds to a condition being present, then the value of that observation is decreased or increased by a constant amount equal to the value of the coefficient of the dummy variable in the regression. • If a condition has three possible values; say “high”, “medium” or “low”. We encode its value with two dummy variables. The first variable, High, equals 1 if the condition is “high” and zero otherwise and the second variable, Medium, equals 1 if the condition is “medium” and zero otherwise. When the condition is “low” both values are zero. The Baseline condition “low” is reflected in the constant in the regression equation. • In time series regression, we use dummy variables for seasons, and use S-1 dummy variables if there are S seasons. We are free to chose the baseline season from which all others are measured. (c) Martin L. Puterman

  7. Trend Regression with Seasonality • My experience suggests that a quadratic trend regression plus (additive) seasonality is useful for forecasting • Uses “dummy variables” for seasons • Must be fit with regression software • Equation with linear trend and additive monthly seasonality Yt = a + bt + dt2+ c2Febt + c3Mart + … + c12Dect • Also enables multiple levels of seasonality such as weekly and monthly. (c) Martin L. Puterman

  8. Trend Regression with Seasonality • In previous Febt, Mart, … are dummy variables • they equal 1 if observation Yt is from the indicated month and 0 otherwise • Note that there is no dummy variable for January • January is the baseline for comparison Examples: Yt = a + bt Observation t in January Yt = a + bt + c2 Observation t in February Yt = a + bt + c3 Observation t in March • In NCSS, declaring a variable as categorical means that NCSS will generate dummy variables automatically • But it will choose the baseline somewhat arbitrarily. (c) Martin L. Puterman

  9. Trend Regression With Seasonality - Example Some forecasts: Jan: F156(1) = 189.88 + 0.36*157 = 246.40 Feb: F156(2) = 189.88 + 0.36*158 - 23.85 = 222.91 Mar: F156(3) = 189.88 + 0.36*159 - 19.96 = 227.16 (c) Martin L. Puterman

  10. Regression Example: Forecast Updating During Season • Goal: Improve total sales forecasts using interim sales data • Data; early forecast, interim sales and total sales data for a wide range of products. • Fitted Model: Total Sales = 120 + .6 Interim Sales +.3 Early Forecast • Example: Early Forecast of Total Sales = 3000; Interim Sales =1400 • Revised Total Sales Forecast • Total Sales = 120 + .6*1400 + .3*3000 = 1860 • Forecast Standard Deviation is Regression RMSE (c) Martin L. Puterman

  11. Regression Example: Impact of Advertising • Goal: Take into account effect of advertising expenditures on sales • Data; Sales and advertising expenditures in previous quarter • Fitted Model: • Salest = 15 + 10 Quartert + .8 Salest-1 +.4 (Advertisingt-1) 1/2 • Example: Sales in last quarter = 2000 and Advertising in previous quarter = 10,000 • Total Sales Forecast • Sales = 15 + .8*2000 + .4*100 =1655 • Forecast Standard Deviation is Regression RMSE (c) Martin L. Puterman

  12. Some special concerns when using regression with time series data • Often the usual regression assumption of uncorrelated errors is violated • This means that the residuals contain information. Case A: This is usually due to model mis-specification; i.e. omission of important variables Case B: But sometimes we have what we think is a good model and there is nothing obvious to add. • Difficulty – Standard errors are underestimated so model seems better than it really is. • Concept: Since observations are not independent, there is less information in the data than you would think • Reject Ho: βj = 0 when we shouldn’t. • Detection • Some systematic pattern in residual plot vs. time • Durbin-Watson Test (see next slide). • (Best approach) ACF of residuals (c) Martin L. Puterman

  13. Durbin-Watson test; comments • The Durbin-Watson test is a not so good alternative to using the ACF of the residuals but it is widely used probably because of historical reasons. • It is based on the Durbin-Watson test statistic D. • It tests only for first order autocorrelation in the errors. • Formally it tests H0:  =0 vs. Ha:  0 • The test is reject H0 and conclude that there is autocorrelation in the residuals if D is well below 2 or well above 2; I suggest being imprecise here. I would worry about values less than 1.4 or greater than 2.6. • In economic data, when  is not zero, it is usually positive. (c) Martin L. Puterman

  14. Regression in the face of correlated residuals • Approaches for obtaining more reliable estimates; • Add variables, such as trend squared, or use the lagged dependent variable as an explanatory variable. (See sales and advertising example on previous slide; Salest-1 is a lagged variable.) • Use time series regression models – which except for a special case (AR1 errors) requires advanced software such as SAS or R. • Use “Multiple regression with serial correlation” under “other regression routines” in NCSS. • Difference data if lag one autocorrelation is large and software such as that above is not available. (c) Martin L. Puterman

  15. Regression with auto-correlated errors Model yt = β0 + β1x1t + ••• + βmxmt + εt where εt = ρ εt-1 + νt and νt ~ N(0, σ2) and independent The quantity ρ is called the first order auto- correlation or serial correlation parameter and is between -1 and +1. The Corchrane-Orcutt procedure, which is coded in NCSS, estimates the regression coefficients and ρ for this model. Note that usually the regression coefficients will not change much from ordinary regression but their standard errors will be larger. (c) Martin L. Puterman

  16. What if seasonality is multiplicative and we want to use regression? • Problem; Model on nominal scale assumes additive effect of seasonal dummy variables. • Solution: Do regression on the logarithmic scale. This means that we transform the dependent variable by taking logarithms (base 10 or base e) and then do regression. • Why does this work? Multiplicative seasonality is additive on the log scale! • Thus we can do forecasts using the model on the log scale and then transform back to the original scale by exponentiating the forecast on the log scale. • Example: If forecast on Log10 scale is 3.4, then forecast on the nominal (original) scale is 103.4 = 2511.9 units. • Also trends and dummy’s on the log-scale have nice interpretations. Consider the model for 4 seasons log10(yt) = 2 + .014t + .19 Season2t -.13 Season3t + .04 Season4t • Then the value of the series is increasing 1.4% per period. • The value in Season2 is about 19% above the what the trend alone predicts for that season. • But predictions based on these transformations are often biased. • Alternative ad hoc approach: Deseasonalize data; fit model to deseasonalized data and then multiply back by seasonal factors to get forecasts. This is how time series decomposition works. (c) Martin L. Puterman

  17. Example – BC IncorporationsTrend Regression Regression Equation Section Regression Standard T-Value Reject Power Independent Coefficient Error to test Prob H0 at of Test Variable b(i) Sb(i) H0:B(i)=0 Level 5%? at 5% Intercept 18672.3162 1865.4549 10.010 0.0000 Yes 1.0000 trend 584.1152 182.0498 3.209 0.0059 Yes 0.8503 Serial Correlation of Residuals Section Serial Serial Serial Lag Correlation Lag Correlation Lag Correlation 1 0.7473 9 -0.2205 17 0.0000 2 0.3632 10 -0.0183 18 0.0000 3 -0.0190 11 0.1487 19 0.0000 4 -0.2897 12 0.2349 20 0.0000 5 -0.4198 13 0.0000 21 0.0000 6 -0.4632 14 0.0000 22 0.0000 7 -0.4478 15 0.0000 23 0.0000 8 -0.3732 16 0.0000 24 0.0000 Above serial correlations significant if their absolute values are greater than 0.485071 Durbin-Watson Test For Serial Correlation Did the Test Reject Parameter Value H0: Rho(1) = 0? Durbin-Watson Value 0.3573 Prob. Level: Positive Serial Correlation 0.0000 Yes Prob. Level: Negative Serial Correlation 1.0000 No (c) Martin L. Puterman

  18. Same data using serial correlation routine Run Summary Section Parameter Value Parameter Value Dependent Variable BC Rows Processed 17 Number Ind. Variables 1 Rows Filtered Out 0 Weight Variable None Rows with X's Missing 0 R2 0.1506 Rows with Weight Missing 0 Adj R2 0.0899 Rows with Y Missing 0 Coefficient of Variation 0.4879 Rows Used in Estimation 17 Mean Square Error 4628200 Sum of Weights 16.000 Square Root of MSE 2151.325 Completion Status Normal Completion Ave Abs Pct Error 17.034 Autocorrelation (Rho) 0.8523 Regression Equation Section Regression Standard T-Value Reject Independent Coefficient Error to test Prob H0 at Variable b(i) Sb(i) H0:B(i)=0 Level 5%? Intercept 10850.3237 12603.3258 0.861 0.4038 No trend 1244.8410 790.0657 1.576 0.1374 No (c) Martin L. Puterman

  19. Same data but adding extra variables Regression Equation Section Regression Standard T-Value Reject Power Independent Coefficient Error to test Prob H0 at of Test Variable b(i) Sb(i) H0:B(i)=0 Level 5%? at 5% Intercept 22290.5091 1303.6382 17.099 0.0000 Yes 1.0000 dummy -37509.5091 7155.5803 -5.242 0.0002 Yes 0.9979 dummyXyear 3036.6909 518.8170 5.853 0.0001 Yes 0.9997 year -73.6909 192.2110 -0.383 0.7076 No 0.0646 • In above – dummy = 1 if year for year > 2001 and dummmyXyear allows for a shift in trends. • Note there is still some autocorrelation present, lag 1 serial autocorrelation equals .32 (which is insignificant) and the Durbin-Watson Test is significant but much less so than without extra variables. • The purpose of this example was to show that autocorrelation can result from the omission of independent variables. (c) Martin L. Puterman

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