5.3 Designing Investigations & Displaying Data
Key Takeaways
- The investigation cycle runs question, hypothesis, method, results, conclusion - and a hypothesis is a testable prediction, often written as "If... then... because..."
- Decide what to measure, with which equipment and in which units, before you start - and design the empty results table first
- Use a table for exact numbers, a bar graph to compare separate groups or categories, and a line graph for continuous change such as growth over time
- A good method is written so clearly - numbered steps, exact amounts and units, named equipment - that another student could repeat it without asking a single question
- To improve a flawed method in a stimulus, find the vague quantity, the missing control, the single trial or the missing unit, and state the exact fix
The Investigation Cycle
Every complete investigation moves through the same five stages, and ICAS questions can target any one of them:
| Stage | What happens | Example (dissolving experiment) |
|---|---|---|
| Question | State what you want to find out, in "How does X affect Y?" form | Does water temperature affect how quickly sugar dissolves? |
| Hypothesis | A hypothesis is a testable prediction, often written "If... then... because..." | If water is hotter, then sugar will dissolve faster, because heat gives the particles more energy |
| Method | Numbered steps saying exactly what to do, keeping it a fair test | Pour 100 mL of water at 20, 40 and 60 degrees into identical beakers, add one teaspoon of sugar to each, stir ten times, time the dissolving |
| Results | The measurements, recorded in a table and often graphed | Sugar dissolved in 95 s at 20 degrees, 48 s at 40 degrees, 21 s at 60 degrees |
| Conclusion | What the results show, linked back to the hypothesis | The results support the hypothesis: hotter water dissolved the sugar faster |
Two habits matter at the conclusion stage. First, a conclusion must come from the data, not from what you hoped would happen - if the results do not support the hypothesis, say so honestly; an unsupported hypothesis is not a failure, it is a finding. Second, a strong conclusion often ends with an evaluation: what could be improved next time, such as more trials or a better-controlled variable.
Choosing Equipment and Measurements
Before collecting any data, decide exactly what you will measure and with what. Match the instrument to the quantity, as covered in the Observing & Measuring chapter: a stopwatch for time in seconds, a thermometer for temperature in degrees Celsius, a balance for mass in grams, a ruler or tape for length. Choose an instrument with a sensible range - a 10 mL measuring cylinder for 5 mL of liquid, not a 1 L jug - and decide your measurement intervals in advance: will you measure the plant every day or every week? Write these decisions into the method before you begin, not after you see the results.
Design the Results Table First
Experienced investigators draw the empty results table before touching any equipment. This forces you to plan what you will change, what you will measure, and how many repeats you will do. The rules:
- The independent variable heads the first column, listing each value you will test.
- The dependent variable heads the later columns, with room for repeated trials and an average.
- Units go in the column headings, in brackets - never inside every cell.
For the dissolving experiment:
| Water temperature (degrees C) | Time to dissolve - Trial 1 (s) | Trial 2 (s) | Trial 3 (s) | Average (s) |
|---|---|---|---|---|
| 20 | ||||
| 40 | ||||
| 60 |
Choosing the Right Display
ICAS loves asking which display suits a set of data. The choice depends on the kind of independent variable:
- Table - exact numbers, especially while collecting or when precision matters.
- Bar graph - when comparing separate categories or groups: mould growth on white, wholemeal and rye bread; rainfall in four cities. The bars do not touch because the categories are separate.
- Line graph - when the independent variable is continuous, especially time: a plant's height measured each week, water cooling minute by minute. The line shows how one thing changes steadily with another.
The quick test: if "in between" values make sense (halfway between week 2 and week 3 there was a height), use a line graph; if the groups are distinct kinds of things, use bars.
Writing a Method Someone Else Could Repeat
Scientific writing has one goal: a stranger must be able to repeat your experiment and get comparable results. Paper G tests these principles directly, asking students to establish the sequence in writing up scientific experiments. A repeatable method has:
- Numbered steps in the order they happen.
- Exact amounts with units - "100 mL", not "some"; "50 g", not "a spoonful".
- Named equipment - "a 250 mL beaker", not "a cup".
- The fair-test details - what is kept the same, and how.
- What to measure, when, and how many times - "measure the height in centimetres every Monday for four weeks".
The acid test: could a classmate follow the method with no chance to ask you anything? If any step needs a guess, rewrite it.
Improving a Flawed Method
Senior ICAS questions describe a flawed method in the stimulus and ask for the best improvement. The usual faults are: a vague quantity ("add some salt"), an uncontrolled second variable, only one trial, or missing units and timings. The best answer is always the fix that targets the actual fault - for example, if every setup differs in two ways, the improvement is to hold one of them constant, not to buy fancier equipment or to repeat the same unfair test more times. Find the fault first, then match the fix.
Every minute for ten minutes, Priya measures the temperature of a cup of hot water as it cools on the bench. She now wants to display her results for her report. Which display suits this data best?
Four students each write one step for an experiment testing whether sugar dissolves faster in hot water. Which step is written clearly enough for another student to repeat exactly?
Ava investigates whether light affects plant growth. She puts one plant on a sunny windowsill and one in a dark cupboard. She waters the windowsill plant every day but waters the cupboard plant only when she remembers. After three weeks the windowsill plant is taller. What is the best improvement to Ava's method?