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
Type your answer and press Check to be marked straight away, or reveal the answer and mark yourself.
Q1On a time series graph, what is always plotted on the horizontal axis?Show answer
Answer: Time (for example, days, months or years).
Q2A time series graph generally slopes upward from left to right over several years. What is this called?Show answer
Answer: An increasing (upward) trend.
Q3A time series shows sales peaking every December for five years in a row. What is this repeating pattern called?Show answer
Answer: Seasonal variation.
Q4A time series of monthly temperatures shows one May with a temperature far below every other May on record. What might this value be called?Show answer
Answer: An unusual value (or an outlier/anomaly) that breaks the seasonal pattern.
Q5A company's yearly profits are 10, 12, 11, 15, 14 thousand pounds over five years. Describe the general trend.Show answer
Answer: A generally increasing trend overall (from 10 up to 14), despite small dips, for example between year 2 and year 3 (12 down to 11) and again between year 4 and year 5 (15 down to 14).
Q6Explain why using a time series trend to estimate a value many years into the future is less reliable than estimating one year ahead.Show answer
Answer: The further into the future the estimate goes, the more the underlying pattern may change, so the projection becomes less certain the further it extends beyond the known data.
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.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 2 available
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.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 3 available
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.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 3 available
See real GCSE Statistics past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the time series worksheet pack - 17 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 13 of GCSE Statistics Foundation Workbook 2 and chapter 18 of GCSE Statistics Higher Workbook 2, the whole course as one free printable PDF.
Next topics
Not quite what you needed?
Tell us what is missing on time series, or which topic to write up next. Every request is read, and we reply to every one.
Build a full practice pack.
This topic is one of hundreds in the library - pick the ones a student needs and generate a printable PDF in minutes.