Running an Investigation
A Year 7 class in Dubbo wants to know one thing: does giving a seedling more sunlight each day make it grow taller? Instead of guessing, they turn it into a real experiment. They change one thing on purpose, keep everything else the same, measure the seedlings every few days, build a results table, draw a line graph, and read the trend. By the end they can answer their question with data, not opinion. In this lesson you will learn how to run a fair investigation from start to finish.
Printable Worksheets
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Q1 · You want to find out if a toy car rolls further when you raise the ramp higher. What one thing would you change on purpose, and what one thing would you measure?
Q2 · Imagine you measure a plant's height only once, on day 1. Why might it be better to measure it every few days instead of just once?
● Know
- The independent variable is the one thing you deliberately change in an investigation (NESA SC4-WS-04)
- The dependent variable is the thing you measure or observe as a result, and controlled variables are everything you keep the same
- Results can be recorded by taking a series of observations over time, then shown in a table and a line graph (NESA SC4-WS-07)
● Understand
- Why changing only the independent variable makes a test fair, so you can trust the result
- How a line graph turns a table of numbers into a picture that shows a trend
- How to read a trend from a graph and write a conclusion that answers the question using the data
● Can do
- Identify the independent, dependent and controlled variables in a simple investigation
- Build a results table and plot a line graph with labelled axes and units
- Describe the trend in a graph and write a clear conclusion from the data
A variable is anything in an investigation that can change or be changed. To run a fair test you need to sort your variables into three kinds. This is the heart of the NSW syllabus idea you are learning today: changing the independent variable produces a change in the dependent variable, in a controlled experiment (SC4-WS-04).
- Independent variable: the one thing you deliberately change. In the seedling investigation, this is the hours of sunlight each plant gets per day. You choose it.
- Dependent variable: the thing you measure or observe as a result. Here it is the height of the seedling in centimetres. It depends on what you changed.
- Controlled variables: everything else you keep the same so the test is fair, such as the type of seed, the size of the pot, the soil, and the amount of water. If these changed too, you could not be sure that sunlight caused any difference.
A handy memory trick: you change the independent variable, you measure the dependent variable, and you keep the same the controlled variables. A test is only fair when just one thing changes at a time.
- Height of the ramp
- Distance the car rolls
- The same toy car each time
- The same smooth floor each time
- Independent variable, the thing they change
- Dependent variable, the thing they measure
- Controlled variable, same car kept for fairness
- Controlled variable, same floor kept for fairness
Many changes do not happen all at once, so a single measurement is not enough. The syllabus tells us to make a series of observations over time (SC4-WS-07). That means you measure the same thing again and again at regular moments, such as every day or every week, and write down each result.
In the Dubbo seedling investigation, the class gives one tray of seedlings 6 hours of sunlight per day. They measure the height of the seedlings every 2 days for 10 days. Each measurement is one observation, and together they form a series that shows how the plant grows.
To keep the data tidy, scientists record it in a results table. A good table has clear column headings, the correct units, and one row for each observation.
| Day | Height (cm) |
|---|---|
| 0 | 2 |
| 2 | 4 |
| 4 | 7 |
| 6 | 9 |
| 8 | 12 |
| 10 | 14 |
Notice how the units, centimetres, go in the column heading, not next to every number. This keeps the table clean and easy to read.
A table is neat, but a line graph turns your numbers into a picture that shows the pattern at a glance. When you measure something over time, a line graph is usually the best choice. Here is how to build one.
- Draw two axes: the horizontal axis (across) and the vertical axis (up).
- Independent variable on the bottom: the thing you changed, here the day, goes on the horizontal axis.
- Dependent variable up the side: the thing you measured, here the height in centimetres, goes on the vertical axis.
- Label each axis with its name and unit, for example "Day" and "Height (cm)".
- Plot each point from your table, then draw a smooth trend line through them to show the overall pattern.
Below is the seedling data from the table, plotted as a line graph. Each dot is one observation, and the line shows the trend.
The line climbs steadily from left to right. That shape tells a story, which you will read in the next card.
The ___ variable, the thing you changed, goes on the horizontal axis along the bottom. The ___ variable, the thing you measured, goes on the vertical axis up the side. Each axis needs a name and a ___, and you draw a ___ line through the plotted points to show the overall pattern.
A trend is the overall pattern in your data, the general direction the line is heading. The syllabus asks you to identify trends and draw conclusions (SC4-WS-07). When you read a trend, you describe how the dependent variable changes as the independent variable changes. There are three common shapes.
- Increasing: as the independent variable goes up, the dependent variable also goes up. The line climbs. (This is what the seedling graph shows.)
- Decreasing: as the independent variable goes up, the dependent variable goes down. The line falls.
- No change: the dependent variable stays about the same no matter what. The line stays flat.
For the seedling graph the trend is clear: as the number of days increases, the height of the seedling increases. The line climbs steadily from 2 cm on day 0 to 14 cm on day 10.
A conclusion is a short sentence that answers the original question using your data. A good conclusion mentions the trend and gives evidence. For example:
"The results show that the seedling grew taller over time. Over the 10 days its height increased from 2 cm to 14 cm, so a series of measurements supports the conclusion that the seedling kept growing."
Notice the conclusion answers the question, names the trend, and uses real numbers from the data. That is exactly what a scientist does at the end of an investigation.
Now you can see how a full investigation fits together. Imagine a class in Wollongong testing this question: does ramp height affect how far a toy car rolls? Here is the whole journey.
- Question: they ask one clear, testable question.
- Variables: the independent variable is ramp height, the dependent variable is the rolling distance, and controlled variables include the same car, the same floor and the same starting point.
- Observations over time: they set the ramp to 10 cm, 20 cm, 30 cm and 40 cm, and measure the distance for each height, repeating to be sure.
- Table: they record each ramp height and distance in a results table with units.
- Graph: they plot ramp height on the bottom axis and distance up the side, then draw a trend line.
- Trend and conclusion: the line climbs, so as ramp height increases, the distance increases. Their conclusion answers the question using the data.
Because they changed only one thing, the ramp height, and kept everything else the same, they can trust that the ramp height caused the change in distance. That is what makes it a controlled experiment.
A class measures the temperature of a cup of hot cordial every 5 minutes as it sits on the bench cooling down. The independent variable is time, and the dependent variable is temperature. Predict: when they plot temperature up the side and time along the bottom, what shape will the trend line have, and what trend will it show?
How close was your prediction?
At the start of the lesson you thought about the toy car and ramp investigation. Now write a complete plan for it.
Your answer must: (1) name the independent variable; (2) name the dependent variable; (3) give two controlled variables; (4) explain how a results table and a line graph would help you find the trend and write a conclusion. Use the words independent, dependent, controlled, trend and conclusion.
Q1. Explain the difference between the independent variable and the dependent variable, then give one example of a controlled variable. (3 marks)
Q2. When you draw a line graph, which variable goes on each axis, and why is it important to label the axes with units? (3 marks)
Q3. A toy car investigation gives these results: ramp 10 cm, distance 30 cm; ramp 20 cm, distance 55 cm; ramp 30 cm, distance 75 cm; ramp 40 cm, distance 95 cm. Describe the trend and write a conclusion that uses the data. (3 marks)
Answers
▾MCQ 1
B. The independent variable is the one thing you deliberately change, here the hours of sunlight. The height is what you measure (dependent), while the soil and pot size are kept the same (controlled).
MCQ 2
C. The dependent variable is the thing you measure as a result, here the height of the plant. The sunlight is what you changed, while the water and seed type are controlled variables.
MCQ 3
D. Keeping the controlled variables the same makes the test fair. Then if only the independent variable changed, you can be sure it caused any change in the result.
MCQ 4
A. The line climbs from 2 cm to 14 cm, so as the number of days increases, the height increases. That is an increasing trend.
MCQ 5
C. The independent variable, the thing you changed, goes on the horizontal axis along the bottom. The dependent variable goes up the vertical axis.
Short Answer 1
Model answer: The independent variable is the one thing you deliberately change in the investigation. The dependent variable is the thing you measure or observe as a result, because it depends on what you changed. A controlled variable is something you keep the same to make the test fair, for example using the same type of seed or the same amount of water each time.
Short Answer 2
Model answer: The independent variable, the thing you changed, goes on the horizontal axis along the bottom. The dependent variable, the thing you measured, goes on the vertical axis up the side. Labelling each axis with its name and unit is important so that anyone reading the graph knows exactly what was measured and in what units, which makes the results clear and trustworthy.
Short Answer 3
Model answer: The trend is increasing: as the ramp height increases, the rolling distance increases. A conclusion using the data: as the ramp was raised from 10 cm to 40 cm, the distance the car rolled increased from 30 cm to 95 cm, so raising the ramp higher made the car roll further.