(More Practice With Trend Forecasts)

Transcription

1 Stats for Strategy HOMEWORK 11 (Topic 11 Part 2) (revised Jan. 2016) DIRECTIONS/SUGGESTIONS You may conveniently write answers to Problems A and B within these directions. Some exercises include special instructions which clarify or modify textbook instructions. Data files are available from the Stats website for some exercises. Text exercises marked with the icon are accessed from the Supplementary Exercises (SE) file posted on the Stats website, not the textbook itself. (More Practice With Trend Forecasts) A. Cable Wire Sales In the last problem of HW10 (Problem E: Chinese Car Ownership), we fit a trend to the time series where the residuals from the trend fit are autocorrelated. In that problem we improved on trend forecasts for future Chinese car ownership by forecasting the residuals, as well. But can you guess how to obtain valid 95% forecasts when the residuals from the trend fit are not autocorrelated? The data file Wire shows U.S. sales (in thousands of feet) of a special flat cable wire used in heavy equipment such as cranes and hoists, from January 2005 through December (a) Make a time-series plot. Do sales appear to follow a linear trend? Answer: (b) Fit a linear trend to the series by simple linear regression. Which variable is the predictor variable? Answer: What is R 2 from this regression? Answer: (c) Consider the sample (fitted) regression equation. Interpret the slope in the space below. (d) Run the regression again, this time storing the residuals in the MINITAB worksheet. Make a time-series plot of the residuals. Add a 0 line by right-clicking inside the plot and Add the Reference Line Y = 0. Apply the Runs Test in MINITAB. Then answer questions on the next page! (continued) 1

2 Write down the hypotheses H 0 and H A (in English) in the space below. What s the P -value? What decision do you recommend, based on the P -value? What s the conclusion? (e) Interpret a 95% forecast for cable wire sales in March (f) Is it necessary to forecast residuals using time-series methods such as Partial Autocorrelation and ARIMA? Why or why not? (continued) 2

3 (Seasonal Forecasts) B. Retail Trade Employment DIRECTIONS: Open the Trade Employment data file and refer to Notebook page 255. First make a plot: Stat > Time Series > Time Series Plot > (Select Trade Employment) > OK Now improve the plot by showing the actual dates along the horizontal axis: (Click the blue box i.e., recall previous commands) > Time/Scale > Stamp > (Select Date) > OK > OK Clearly, these data show both a trend and seasonality. The first step is to model the trend with simple regression. So make a Fitted Line Plot! Also get the full regression output for Trade vs. Month and make a MINITAB prediction for Month 80, as shown on Notebook page 256. Did you reproduce the plot and output? To see the autocorrelation of residual errors from simple regression, rerun Regression... Fit Regression Model and store the residuals in the MINITAB worksheet: Storage > (Select Residuals) > OK > OK Now make a scatterplot of Residuals vs. Month: Graph > Scatterplot... Since this is a time series we should connect the dots to reproduce Figure 2: Graph > Scatterplot > With Connect Line > OK... Just for fun add a horizontal line to the graph to separate positive residuals from negative residuals: (Right-click inside the graph) > Add > Reference Lines > Y = 0 > OK Answer these questions using the Trend-Only model: (a) Use MINITAB Regression Predict to predict trade employment in March (b) Predict trade employment in March 1976 with 95% certainty. (continued) 3

4 B. continued DIRECTIONS: The residuals plot from simple regression shows clear seasonality: Notice that the errors spike in December of each year (at months 12, 24, 36,....) So let s improve the model by incorporating seasonality as well as trend. Reproduce the regression output and predictions shown on Notebook page 259: Stat > Regression > Regression > Fit Regression Model > (Select Response Trade Employment) > (Select Continuous Predictors Month S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 S 11 ) Stat > Regression... Predict > (Enter ) Again plot Residuals vs. Month for your new model. Did you reproduce Figure 3 on page 261? Answer these questions using the Trend-and-Seasonal model: (c) Predict trade employment in March (d) Predict trade employment in March 1976 with 95% certainty. (e) Estimate by how much trade employment in December exceeds trade employment in January, after accounting for trend. (f) Estimate by how much trade employment in December exceeds trade employment in June, after accounting for trend. (g) Estimate the average increase in trade employment from May to June of each year. (continued) 4

5 (Improving Seasonal Forecasts by Modeling Autocorrelation) B. continued DIRECTIONS: Open to Notebook page 265. Reproduce MINITAB Steps 1 5 exactly as shown on pages , checking at each step that you produce the output in the Notes. Review the reasoning at each step. Questions: (h) How many periods (months) of past Trade Employment residuals affect future Trade Employment residuals, using 5% significance for statistical tests? (Find p = # periods for the AR(p) models described on Notes page 264.) Answer: (i) The original data ended in December 1974 and in the Notes we produced forecasts for Jan. June 1975 (Months ) Now modify the MINITAB commands to produce both a simple forecast and a 95% interval forecast for Trade Employment in December (j) Produce both a simple forecast and a 95% forecast for March (end of Problem B) 5

6 C. JCPenny Retail Sales (Note: Please use your own scratch paper for Problem C.) Supplementary Exercise 13.1 (SE file p. 131) (b) Tip: Use the Time/Scale option to put actual dates on the horizontal axis of the time-series plot. Supplementary Exercise 13.2 Supplementary Exercise 13.3 Tip: This exercise requires indicator variables to represent seasonality in quarterly data. To do this, you can simply enter 1 s and 0 s by hand in three new columns: { 1 for first quarter x 1 = 0 otherwise { 1 for second quarter x 2 = 0 otherwise { 1 for third quarter x 3 = 0 otherwise Quarter x 1 x 2 x With this setup, the three seasonal coefficients always compare to the fourth quarter. After you add the three dummy variables for Quarters 1 3, your MINITAB worksheet should look like this: Order Year-Quarter Sales Q1 Q2 Q st nd rd th st nd rd th st nd rd th st nd rd th st nd rd th st nd rd th Supplementary Exercise 13.5 Supplementary Exercise

7 C. continued Supplementary Exercise 13.9 You ll need to consider MINITAB output for both the Trend-Only and Trend-Seasonal regression models. Tip: Use this MINITAB trick to get both the Trend-Only and Trend-Seasonal predictions: Storage > (Check Fits) Then use Time Series > Time Series Plot > Multiple > OK to plot the three time series together, as the textbook requests in part (c). Supplementary Exercise Special instructions: Do part (a) only. Supplementary Exercise Additional Exercise for Problem C We concluded that the Trend-Seasonal model is superior to the Trend-Only model so we ll make forecasts using the Trend-Seasonal model. Follow these steps: Store residuals from the Trend-Seasonal model. Fit an AR(p) autoregression model to the residuals using MINITAB Partial Autocorrelation and ARIMA procedures. Questions: (a) Find the correct value of p for the AR(p) model, using 5% significance. (b) From MINITAB output, write down the fitted AR(p) model for residuals. (c) Use MINITAB to make both a simple forecast and a 95% forecast for JCPenny sales in the first quarter of (d) Make both a simple forecast and a 95% forecast for the fourth quarter of (e) Make both a simple forecast and a 95% forecast for the third quarter of (f) Explain why it makes sense for the residual forecasts ê t for the JCPenny data to be negative numbers. (g) Statistically speaking, is it easier or harder to forecast sales for the third quarter of 2003 compared to the fourth quarter of 2002? Cite numerical evidence to support your answer. 7

8 (Seasonal Factors and Deseasonalized Data) C. continued DIRECTIONS: Review our work with seasonal factors and deseasonalized data for the Trade Employment data Notebook pages Let the ideas settle in. Supplementary Exercise 13.4 Tip for part (c): Type the four seasonal factors into a MINITAB column and make a connected scatterplot. Add part (d): (d) Use MINITAB Time Series Decomposition. Are MINITAB s seasonal factors similar to the ones which you calculated in (a)? Are they identical? Plot the seasonally-adjusted time series. What are seasonally-adjusted JCPenny sales in the second quarter of 1998? Supplementary Exercise 13.7 (end of Problem C) 8

9 D. Number of Macs Shipped (Note: You ll need scratch paper for Problem D.) Exercise 13.8 (p. 697) (a) Enter data in a MINITAB worksheet and use Stat > Time Series > Time Series Plot and choose Fiscal Year/Qtr as the stamp variable in the Time/Scale option to plot actual dates on the horizontal axis. (b) As practice for the final exam, use simple linear regression calculation formulas for b 0 and b 1. (See Exam 3 formula sheet.) Warning: The textbook s table of statistics for the time predictor variable t = Quarter and the response variable Macs Shipped contains errors! So instead, use the information below obtained from MINITAB Descriptive Statistics and Correlation procedures: Descriptive Statistics: Quarter, Units shipped Variable N N* Mean SE Mean StDev Minimum Q1 Median Q3 Quarter Units shipped Variable Maximum Quarter Units shipped Correlations: Quarter, Units shipped Pearson correlation of Quarter and Units shipped = (c) Tip: Use Stat > Time Series > Trend Analysis > (Select Macs-Shipped) > OK to make the plot. Add part (e): (e) How does the trend line from the MINITAB plot from (c) compare to the regression line which you computed in (b)? Exercise (p. 704) Exercise (p. 706) Add part (d): (d) Use MINITAB Time Series Decomposition. Are MINITAB s seasonal factors similar to the ones which you calculated in (a)? Are they identical? Plot the seasonally-adjusted time series. shipments in the first quarter of 2008? What are seasonally-adjusted Mac (end of assignment) 9