Charles tyrwhitt turnover

Statistics, Data Analysis, and Decision Modeling

FOURTH EDITION

James R. Evans

9780558689766

Chapter 7 Forecasting

Introduction

QUALITATIVE AND JUDGMENTAL METHODS

Historical Analogy

The Delphi Method

Indicators and Indexes for Forecasting

STATISTICAL FORECASTING MODELS

FORECASTING MODELS FOR STATIONARY TIME SERIES

Moving Average Models

Error Metrics and Forecast Accuracy

Exponential Smoothing Models

FORECASTING MODELS FOR TIME SERIES WITH TREND AND SEASONALITY

Models for Linear Trends

Models for Seasonality

Models for Trend and Seasonality

CHOOSING AND OPTIMIZING FORECASTING MODELS USING CB PREDICTOR

REGRESSION MODELS FOR FORECASTING

Autoregressive Forecasting Models

Incorporating Seasonality in Regression Models

Regression Forecasting with Causal Variables

THE PRACTICE OF FORECASTING

BASIC CONCEPTS REVIEW QUESTIONS

SKILL-BUILDING EXERCISES

SKILL-BUILDING EXERCISES

PROBLEMS AND APPLICATIONS

CASE: ENERGY FORECASTING

APPENDIX: ADVANCED FORECASTING MODELS—THEORY AND COMPUTATION

Double Moving Average

Double Exponential Smoothing

Additive Seasonality

Multiplicative Seasonality

Holt–Winters Additive Model

Holt– –Winters Multiplicative Model

INTRODUCTION

One of the major problems that managers face is forecasting future events in order to make good decisions. For example, forecasts of interest rates, energy prices, and other economic indicators are needed for financial planning; sales forecasts are needed to plan production and workforce capacity; and forecasts of trends in demographics, consumer behavior, and technological innovation are needed for long-term strategic planning. The government also invests significant resources on predicting short-run U.S. business performance using the Index of Leading Indicators. This index focuses on the performance of individual businesses, which often is highly correlated with the performance of the overall economy, and is used to forecast economic trends for the nation as a whole. In this chapter, we introduce some common methods and approaches to forecasting, including both qualitative and quantitative techniques.

Managers may choose from a wide range of forecasting techniques. Selecting the appropriate method depends on the characteristics of the forecasting problem, such as the time horizon of the variable being forecast, as well as available information on which the forecast will be based. Three major categories of forecasting approaches are qualitative and judgmental techniques, statistical time-series models, and explanatory/causal methods.

Qualitative and judgmental techniques rely on experience and intuition; they are necessary when historical data are not available or when the decision maker needs to forecast far into the future. For example, a forecast of when the next generation of a microprocessor will be available and what capabilities it might have will depend greatly on the opinions and expertise of individuals who understand the technology.

Statistical time-series models find greater applicability for short-range forecasting problems. A time series is a stream of historical data, such as weekly sales. Time-series models assume that whatever forces have influenced sales in the recent past will continue into the near future; thus, forecasts are developed by extrapolating these data into the future.

Explanatory/causal models seek to identify factors that explain statistically the patterns observed in the variable being forecast, usually with regression analysis. While time-series models use only time as the independent variable, explanatory/causal models generally include other factors. For example, forecasting the price of oil might incorporate independent variables such as the demand for oil (measured in barrels), the proportion of oil stock generated by OPEC countries, and tax rates. Although we can never prove that changes in these variables actually cause changes in the price of oil, we often have evidence that a strong influence exists.

Surveys of forecasting practices have shown that both judgmental and quantitative methods are used for forecasting sales of product lines or product families, as well as for broad company and industry forecasts. Simple time-series models are used for short- and medium-range forecasts, whereas regression analysis is the most popular method for long-range forecasting. However, many companies rely on judgmental methods far more than quantitative methods, and almost half judgmentally adjust quantitative forecasts.

In this chapter, we focus on these three approaches to forecasting. Specifically, we will discuss the following:

Historical analogy and the Delphi method as approaches to judgmental forecasting

Moving average and exponential smoothing models for time-series forecasting, with a discussion of evaluating the quality of forecasts

A brief discussion of advanced time-series models and the use of Crystal Ball (CB) Predictor for optimizing forecasts

The use of regression models for explanatory/causal forecasting

Some insights into practical issues associated with forecasting

Qualitative and Judgmental Methods

Qualitative, or judgmental, forecasting methods are valuable in situations for which no historical data are available or for those that specifically require human expertise and knowledge. One example might be identifying future opportunities and threats as part of a SWOT (Strengths, Weaknesses, Opportunities, and Threats) analysis within a strategic planning exercise. Another use of judgmental methods is to incorporate nonquantitative information, such as the impact of government regulations or competitor behavior, in a quantitative forecast. Judgmental techniques range from such simple methods as a manager’s opinion or a group-based jury of executive opinion to more structured approaches such as historical analogy and the Delphi method.