Time Series
A time series is a set of data values recorded at successive points or intervals in time, such as monthly sales figures or yearly rainfall totals, plotted on a graph with time along the horizontal axis. In GCSE Statistics, time series graphs are used to spot trends, seasonal patterns and unusual values as data changes over time.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Plot each data value against the time it was recorded, with time always on the horizontal axis.
- Join consecutive points with straight lines, in time order, to show how the value changes over time.
- Look at the overall direction of the graph to describe the trend, such as generally increasing, decreasing or staying roughly constant.
- Look for a repeating pattern that occurs at regular intervals, such as every quarter or every year, which is called seasonal variation.
- Identify any points that break the general pattern noticeably, since these may be unusual or one-off values worth commenting on.
- Use the trend and any seasonal pattern to make a cautious estimate of a future value, rather than assuming exact repetition.
Worked example
A shop records its quarterly sales, in thousands of pounds, over two years: Year 1: Q1 20, Q2 28, Q3 35, Q4 22; Year 2: Q1 24, Q2 32, Q3 39, Q4 26. Describe the trend and the seasonal pattern shown by this time series.
- Compare the same quarter across both years: Q1 rises from 20 to 24, Q2 rises from 28 to 32, Q3 rises from 35 to 39, and Q4 rises from 22 to 26.
- Since every matching quarter is higher in Year 2 than in Year 1, the overall trend is increasing.
- Compare the quarters within each year: Q3 is the highest value and Q1 is the lowest value in both years.
- Since this same pattern, a peak in Q3 and a dip in Q1, repeats every year, this is the seasonal variation.
- Final answer: the time series shows an increasing overall trend, with a repeating seasonal pattern of a peak in Q3 and a dip in Q1 each year.
Practice questions
Try each question, then tap to reveal the answer.
Exam-style questions
Written in the style of a GCSE Statistics exam paper, with a full mark scheme.
A time series graph shows a company's website visitors each month for three years, generally rising over time but with a dip every August. Describe the trend and identify the type of pattern shown by the August dip.
The table shows a cafe's quarterly ice cream sales, in units, over two years: Year 1: Q1 40, Q2 90, Q3 150, Q4 60; Year 2: Q1 48, Q2 100, Q3 165, Q4 68. State, with a reason, which quarter shows the highest sales, and describe how Year 2 compares with Year 1 overall.
A charity's annual donations, in thousands of pounds, for five consecutive years are 45, 52, 48, 60, 63. A trustee wants to estimate donations for next year using this time series. State an appropriate estimate, and explain one reason why this estimate might not be accurate.
Free printable worksheet
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