Module H3 Practical 03
Estimating Seasonal Effects
Objectives:
By the end of this practical you should be able to:
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Derive seasonal effects using an additive model for a given time series
Examine trends in the series after it is deseasonalised
Interpret findings from the above analyses
1. The example of unemployed school leavers was used in the lecture to illustrate how the
seasonal effects may be estimated using a multiplicative model. In this exercise, use the
same data, available in the worksheet named Leavers in the Excel file H3_data.xls , to
estimate seasonal effects using an additive model.
Objective:
To estimate the seasonal effects in the series so that the seasonal effect can be removed in
studying the general trend in the time series.
(a) Working in pairs, follow the procedures discussed in the lecture to estimate the seasonal
effects using an additive model. Carefully note down all steps you undertake in this
process.
SADC Course in Statistics
Module H3 Practical 03 – Page 1
Module H3 Practical 03
(b) Use your estimates to deseasonalise the series and plot this with the raw data. Examine
the presence of trends in the series and comment on your findings.
(c) Write a short summary of the key lessons you learnt from the above analysis.
SADC Course in Statistics
Module H3 Practical 03 – Page 2
Module H3 Practical 03
2. This second exercise is aimed at consolidating ideas covered in the lecture and
knowledge gained in the first exercise of this practical.
Go into CAST for SADC: Higher Level.
Click on Section 2. Time Series. You will see three sub-titles as:
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2.1 Trends and forecasting
2.2 Cyclical pattern
2.3 Seasonal data
In this practical, you will be using sub-section 2.3.
Read and work through sections on
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Page 1: Seasonal Patterns
Page 2: Smoothing out seasonal variation
Page 3: Estimating seasonal effect
Page 4: Deseasonalised data
Page 5: Putting it all together
Note down the particular time series concepts you learnt while doing this practical.
Comment on features that you found easy or particularly difficult during your work above.
SADC Course in Statistics
Module H3 Practical 03 – Page 3