Interpreting a dataset means doing three things in order: read the numbers accurately, process them (a mean, a difference or a percentage), and then write a conclusion that matches the evidence and no more. In IGCSE Chemistry the data is always supplied in the question, so you never need to remember real pollution figures.
This lesson follows telling dissolved substances from suspensions and leads into comparing a claim with its evidence. The whole module is at water, air and environmental chemistry.
How should you approach a table of readings?
Work through the same sequence each time.
- Read the headings and units first. A column of numbers means nothing until you know what is measured and in which unit.
- Find the pattern. Is the value higher at one site? Does it rise or fall with distance?
- Process the numbers if the question asks: mean, difference, ratio or percentage change.
- Write the conclusion using words that match the strength of the evidence.
- State a limit if the question asks for evaluation: how many samples, how many days, what was not measured.
Worked example
All data below is invented for teaching. A student measures nitrate in river water at three sites on one morning. Units are mg/dm³.
| Site | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| Upstream | 2.0 | 2.2 | 2.1 |
| Farm drain | 18.0 | 18.8 | 18.4 |
| Downstream | 9.1 | 9.5 | 9.0 |
Step 1, mean at each site.
- Upstream: (2.0 + 2.2 + 2.1) ÷ 3 = 6.3 ÷ 3 = 2.1
- Farm drain: (18.0 + 18.8 + 18.4) ÷ 3 = 55.2 ÷ 3 = 18.4
- Downstream: (9.1 + 9.5 + 9.0) ÷ 3 = 27.6 ÷ 3 = 9.2
Step 2, compare. The farm drain mean is 18.4 ÷ 2.1 = 8.76, so it is about 8.8 times the upstream mean.
Step 3, describe the pattern. Nitrate is lowest upstream, highest in the farm drain and lower again downstream.
Step 4, conclusion. The data suggests the farm drain adds nitrate to the river. Downstream is lower than the drain, which fits the drain water being diluted by the river.
Step 5, limit. All readings were taken on one morning at three places. Other sources of nitrate were not measured, so the data does not prove the farm is the cause.
A second dataset is often given.
Invented: dissolved oxygen is 8.0 mg/dm³ upstream and 4.5 mg/dm³ downstream. The decrease is 8.0 − 4.5 = 3.5. The percentage decrease is 3.5 ÷ 8.0 × 100 = 43.75%, so about 44%.
The mistake to watch for
Mistaken answer: “This proves the farm is polluting the river.”
The data is a pattern at one time and place. A pattern shows association, not cause. Other sources, such as a drain from housing, could give the same readings.
The correction is to use “suggests” and then name what extra evidence would help: readings on more days, readings on the farm’s own drain with and without fertiliser use, and other possible sources measured as well.
Another slip is forgetting the unit or dividing by the wrong number when finding a mean. Always divide by the number of readings, which is 3 here, not by the sum.
Check yourself
1. Invented data: phosphate readings at a lake inlet are 0.40, 0.44 and 0.42 mg/dm³. Find the mean.
Show answer
0.40 + 0.44 + 0.42 = 1.26. Dividing by 3 gives 0.42 mg/dm³.
2. The phosphate reading at the lake outlet has a mean of 0.21 mg/dm³. Calculate the percentage decrease from the inlet mean of 0.42.
Show answer
Decrease = 0.42 − 0.21 = 0.21. Then 0.21 ÷ 0.42 = 0.5, so the decrease is 50%.
3. Which word is better in a conclusion from one day’s readings at one site: “proves” or “suggests”? Give a reason.
Show answer
“Suggests.” One day and one site show a pattern but cannot rule out other explanations, so the data cannot prove a cause.
Where this leads next
Carry on with tracing a carbon-containing process, where the evidence becomes a balanced equation and a calculation. The mistake log and retest queue helps you track whether you repeat the same interpretation slip. Then try the module practice set.
Conclusions that go slightly beyond the data are one of the hardest habits to spot alone. In online one-to-one Chemistry tuition, our teachers read your written conclusions with you and point to the exact phrase that overreaches.