Bivariate Time-Series Graphs

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📊 Part 1: Understanding Bivariate Time-Series Graphs

1. What is bivariate time-series data?

Data showing one variable over time

Data showing two variables measured at the same time points

Data showing variables without considering time

Data collected only once

2. On a time-series graph, what does the horizontal (X) axis typically represent?

The first variable being measured

The second variable being measured

Time (days, months, years, etc.)

The scale of measurement

3. Which of the following are important features when creating a bivariate time-series graph? (Select all that apply)

Clear labels on both axes

A legend or key to distinguish the two variables

Different colours or line styles for each variable

Only plotting one variable at a time

4. Look at the graph below showing monthly temperature (°C) and rainfall (mm) for Auckland. In which month did Auckland have the highest temperature?

Note: Imagine a graph where temperature peaks in January at 24°C and rainfall is lowest in February at 15mm.

January

February

July

December

📈 Part 2: Interpreting Bivariate Time-Series Data

Use the following scenario for questions 5-8:
A Year 7 student tracked their daily steps and hours of sleep over two weeks. The graph shows that steps ranged from 8,000 to 15,000 per day, while sleep ranged from 6 to 10 hours per night.

5. Describe one trend you might observe if someone gets more sleep on nights before they take more steps the next day.

6. What type of relationship would you expect between hours of sleep and daily steps for a healthy lifestyle?

As sleep increases, steps decrease

As sleep increases, steps increase (up to a point)

No relationship between sleep and steps

Steps are always the same regardless of sleep

7. If you saw that on day 5, both sleep hours and daily steps were at their lowest point, what might you conclude?

8. Complete this sentence using appropriate mathematical vocabulary:

When both variables show an upward _____________ over time, we can say they have a positive _____________ during that period.

🎯 Part 3: Analysis and Critical Thinking

9. A school wants to investigate whether there's a relationship between daily temperature and the number of students who buy ice blocks from the canteen. Suggest two time periods when this data collection would be most useful:

10. Look at this statement: "The graph shows that higher temperatures always cause students to buy more ice blocks." Explain why this conclusion might be too simple, even if the data shows both variables increasing together.

11. Create your own example of bivariate time-series data that would be interesting to investigate at your school or in your community. Include:

• The two variables you would measure: _________________ and _________________

• The time period: _________________________________

• One prediction about the relationship: ___________________________________________