Autocorrelatie van seizoens- en cyclische tijdreeksen

Als gegevens seizoensgebonden of cyclisch zijn, piekt de ACF rond de seizoensvertragingen of bij de gemiddelde cycluslengte.

Je gaat dit verschijnsel onderzoeken door de jaarlijkse zonnevlekreeks (die de zonnecyclus van ongeveer 10–11 jaar volgt) in sunspot.year en het dagelijkse verkeer naar de Hyndsight-blog (dat een wekelijkse 7-daagse patroon volgt) in hyndsight te plotten. Beide objecten zijn in je werkruimte geladen.

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Maak een tijdplot en een ACF-plot van sunspot.year.
Kijk in de ACF-plot: bij welke vertraging (x) vind je de maximale autocorrelatie (y)? Stel deze gelijk aan maxlag_sunspot.
Maak een tijdplot en een ACF-plot van hyndsight.
Kijk in de ACF-plot: bij welke vertraging (x) vind je de maximale autocorrelatie (y)? Stel deze gelijk aan maxlag_hyndsight.

Praktische interactieve oefening

Probeer deze oefening eens door deze voorbeeldcode in te vullen.

# Plot the annual sunspot numbers
autoplot(___)
ggAcf(___)

# Save the lag corresponding to maximum autocorrelation
maxlag_sunspot <- ___

# Plot the traffic on the Hyndsight blog
___
___

# Save the lag corresponding to maximum autocorrelation
maxlag_hyndsight <- ___

Code bewerken en uitvoeren

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The first thing to do in any data analysis task is to plot the data. Graphs enable many features of the data to be visualized, including patterns, unusual observations, and changes over time. The features that are seen in plots of the data must then be incorporated, as far as possible, into the forecasting methods to be used.

Exercise 1: Welkom bij Forecasting met R Exercise 2: Tijdreeksobjecten maken in R Exercise 3: Tijdreeksplots Exercise 4: Seizoensplots Exercise 5: Trends, seizoenspatronen en cycliciteit Exercise 6: Autocorrelatie van niet-seizoensgebonden tijdreeksen Exercise 7: Autocorrelatie van seizoens- en cyclische tijdreeksen

Huidige oefening

Exercise 8: Koppel de ACF aan de tijdreeks Exercise 9: Witte ruis Exercise 10: Aandelenkoersen en witte ruis

In this chapter, you will learn general tools that are useful for many different forecasting situations. It will describe some methods for benchmark forecasting, methods for checking whether a forecasting method has adequately utilized the available information, and methods for measuring forecast accuracy. Each of the tools discussed in this chapter will be used repeatedly in subsequent chapters as you develop and explore a range of forecasting methods.

Exercise 1: Forecasts and potential futures Exercise 2: Naive forecasting methods Exercise 3: Fitted values and residuals Exercise 4: Checking time series residuals Exercise 5: Training and test sets Exercise 6: Evaluating forecast accuracy of non-seasonal methods Exercise 7: Evaluating forecast accuracy of seasonal methods Exercise 8: Do I have a good forecasting model?Exercise 9: Time series cross-validation Exercise 10: Using tsCV() for time series cross-validation

Forecasts produced using exponential smoothing methods are weighted averages of past observations, with the weights decaying exponentially as the observations get older. In other words, the more recent the observation, the higher the associated weight. This framework generates reliable forecasts quickly and for a wide range of time series, which is a great advantage and of major importance to applications in business.

Exercise 1: Exponentially weighted forecasts Exercise 2: Simple exponential smoothing Exercise 3: SES vs naive Exercise 4: Exponential smoothing methods with trend Exercise 5: Holt's trend methods Exercise 6: Exponential smoothing methods with trend and seasonality Exercise 7: Holt-Winters with monthly data Exercise 8: Holt-Winters method with daily data Exercise 9: State space models for exponential smoothing Exercise 10: Automatic forecasting with exponential smoothing Exercise 11: ETS vs seasonal naive Exercise 12: Match the models to the time series Exercise 13: When does ETS fail?

ARIMA models provide another approach to time series forecasting. Exponential smoothing and ARIMA models are the two most widely-used approaches to time series forecasting, and provide complementary approaches to the problem. While exponential smoothing models are based on a description of the trend and seasonality in the data, ARIMA models aim to describe the autocorrelations in the data.

Exercise 1: Transformations for variance stabilization Exercise 2: Box-Cox transformations for time series Exercise 3: Non-seasonal differencing for stationarity Exercise 4: Seasonal differencing for stationarity Exercise 5: ARIMA models Exercise 6: Automatic ARIMA models for non-seasonal time series Exercise 7: Forecasting with ARIMA models Exercise 8: Comparing auto.arima() and ets() on non-seasonal data Exercise 9: Seasonal ARIMA models Exercise 10: Automatic ARIMA models for seasonal time series Exercise 11: Exploring auto.arima() options Exercise 12: Comparing auto.arima() and ets() on seasonal data

The time series models in the previous chapters work well for many time series, but they are often not good for weekly or hourly data, and they do not allow for the inclusion of other information such as the effects of holidays, competitor activity, changes in the law, etc. In this chapter, you will look at some methods that handle more complicated seasonality, and you consider how to extend ARIMA models in order to allow other information to be included in the them.

Exercise 1: Dynamic regression Exercise 2: Forecasting sales allowing for advertising expenditure Exercise 3: Forecasting electricity demand Exercise 4: Dynamic harmonic regression Exercise 5: Forecasting weekly data Exercise 6: Harmonic regression for multiple seasonality Exercise 7: Forecasting call bookings Exercise 8: TBATS models Exercise 9: TBATS models for electricity demand Exercise 10: Your future in forecasting!