Forecasting: principles and practice. Welcome to our online textbook on forecasting.
This textbook is intended to provide a comprehensive introduction to forecasting methods and to present enough information about each method for readers to be able to use them sensibly. We don’t attempt to give a thorough discussion of the theoretical details behind each method, although the references at the end of each chapter will fill in many of those details. The book is written for three audiences: (1) people finding themselves doing forecasting in business when they may not have had any formal training in the area; (2) undergraduate students studying business; (3) MBA students doing a forecasting elective.
We use it ourselves for a second-year subject for students undertaking a Bachelor of Commerce degree at Monash University, Australia. For most sections, we only assume that readers are familiar with algebra, and high school mathematics should be sufficient background. 13 Resources. Time series - AR(1) selection using sample ACF-PACF. Forecasting within limits. Forecasting within limits It is common to want forecasts to be positive, or to require them to be within some specified range .
Both of these situations are relatively easy to handle using transformations. Positive forecasts To impose a positivity constraint, simply work on the log scale. . Interpreting noise. When watching the TV news, or reading newspaper commentary, I am frequently amazed at the attempts people make to interpret random noise.
For example, the latest tiny fluctuation in the share price of a major company is attributed to the CEO being ill. When the exchange rate goes up, the TV finance commentator confidently announces that it is a reaction to Chinese building contracts. No one ever says “The unemployment rate has dropped by 0.1% for no apparent reason.” What is going on here is that the commentators are assuming we live in a noise-free world. They imagine that everything is explicable, you just have to find the explanation. The finance news Every night on the nightly TV news bulletins, a supposed expert will go through the changes in share prices, stock prices indexes, currency rates, and economic indicators, from the past 24 hours.
Errors on percentage errors. The MAPE (mean absolute percentage error) is a popular measure for forecast accuracy and is defined as where denotes an observation and denotes its forecast, and the mean is taken over Armstrong (1985, p.348) was the first (to my knowledge) to point out the asymmetry of the MAPE saying that “it has a bias favoring estimates that are below the actual values”.
Modelling seasonal data with GAMs. In previous posts I have looked at how generalized additive models (GAMs) can be used to model non-linear trends in time series data.
At the time a number of readers commented that they were interested in modelling data that had more than just a trend component; how do you model data collected throughout the year over many years with a GAM? In this post I will show one way that I have found particularly useful in my research. First an equation. Monthly seasonality. I regularly get asked why I don’t consider monthly seasonality in my models for daily or sub-daily time series.
For example, this recent comment on my post on seasonal periods, or this comment on my post on daily data. The fact is, I’ve never seen a time series with monthly seasonality, although that does not mean it does not exist. Monthly seasonality would occur if there is some regular activity that takes place every month and which affects the time series. For example, some companies try to average their expenditure across the month and often have to spend more at the end of the month to justify the budget.
So daily expenditure tends to increase at the end of each month, producing a monthly seasonal pattern. Or imagine a situation where a company always stocks up on supplies on the second Tuesday in every month. This type of seasonal pattern is quite difficult to model. Rmnppt/FruitAndVeg - HTML - GitHub. Monitoring Count Time Series in R: Aberration Detection in Public Health Surveillance. Why time series forecasts prediction intervals aren't as good as we'd hope. Five different sources of error When it comes to time series forecasts from a statistical model we have five sources of error: Random individual errors Random estimates of parameters (eg the coefficients for each autoregressive term) Uncertain meta-parameters (eg number of autoregressive terms) Unsure if the model was right for the historical data Even given #4, unsure if the model will continue to be right A confidence interval is an estimate of the statistical uncertainty of the estimated parameters in the model.
It usually estimates the uncertainty source #2 above, not interested in #1 and conditional on the uncertainty of sources #3, #4 and #5 all being taken out of the picture. A prediction interval should ideally take all five sources into account (see Rob Hyndman for more on the distinction between prediction and confidence intervals). Here’s a simple simulation to show the cost of estimating the meta-parameters, even when sources of error #4 and #5 can be discounted.