ACARA v9 CONTENT DESCRIPTION “compare methods and findings with those of others, recognise possible sources of error, pose questions for further investigation and select evidence to draw reasoned conclusions”
Builds on running a fair test and reading a graph. Two groups set out to find which condition rusts iron nails fastest. Each group left identical nails in water, in salty water and in dry air, then measured the rust. They followed slightly different methods and ended up with different findings. The job now is to compare those methods and findings, recognise a likely source of error, select the evidence that holds up, and pose a sharper question to investigate next.
Compare methods, not just findings
When two groups investigate the same thing and get different answers, the first move is to line up what each group actually did. Both groups put iron nails in water, salty water and dry air, and both weighed the rust that formed. Yet their numbers do not match. That gap is a clue, not a contest. Group A dried every nail before weighing it; Group B weighed the nails while they were still wet. Comparing the methods often explains the gap and tells you which findings you can trust.
Two groups' rust amounts side by side
Both groups left iron nails in five conditions for a week and weighed the rust in milligrams. Switch between the table, bars and line to compare their findings.
My group's rust amounts, with every nail dried before weighing. The other group, who weighed their nails while still wet, recorded 5, 14, 30, 53 and 70 mg. Both sets climb in the same order: dry air rusts least and salty warm water rusts most. The groups agree on the pattern even though the other group's numbers all run higher, which is the first sign that a method difference, not the rust itself, is behind the gap.
Recognise a possible source of error
Before trusting a comparison, look for a source of error: something that pushes a measurement away from the true value. Here one group weighed nails while they were still wet, so the leftover water added to every recorded mass. Other sources of error could be running the test for a different number of days, starting with nails of different sizes, or knocking loose flakes of rust off before weighing. When findings disagree, an unrecognised source of error is often the reason, so it is the first thing to hunt for.
Find the reading with a source of error
Five students each weighed the rust on an identical nail kept in the same salty water for the same week, drying it first. Four readings sit close together, but one is far off. Click the reading that does not match the others.
Click the point that does not fit the pattern of the others.
Weigh the method differences
Not every difference between the two methods matters the same amount. Some differences are the real source of error behind the mismatched findings; others barely change the result. Weigh each method difference to decide which one most likely caused the gap. The one that changes what the scale actually measures is the one to fix in the next test.
Which method difference most likely caused the gap?
The two groups did three things differently. Pick each one to see what it gains in convenience and what it costs in fairness, then decide which is the real source of error.
Group A and Group B both rusted the same kind of nails in the same conditions, but their methods differed in three ways. Choosing each difference shows what it gives and what it gives up when you try to compare the findings fairly.
Choose a response to see what is gained and what is given up.
Select evidence for a reasoned conclusion
A reasoned conclusion rests only on the evidence that holds up. The masses taken from still-wet nails cannot be compared fairly, so that evidence is set aside. What remains is what both groups found the same way: the nails rusted most in salty warm water and barely rusted in dry air, with the amount climbing in the same order for both groups. Sort which statements the surviving evidence supports, and which should be left out.
Which evidence supports the reasoned conclusion?
The conclusion should rest only on evidence both groups can stand behind. Decide which statements the rust measurements actually support.
Claim: Iron nails rust fastest in salty, warm water and barely rust in dry air.
Both groups found the most rust on nails kept in salty warm water.
Both groups found almost no rust on nails kept in dry air.
In both groups, the rust amount climbed in the same order across the five conditions.
One group weighed its nails while still wet, so those mass readings ran high and cannot be compared fairly.
Our group used clear plastic jars, which we found easiest to see through.
Decide whether each statement is evidence for the claim, or not.
Why this matters
Comparing methods as well as findings, recognising where error creeps in, selecting the evidence that holds up, and posing a sharper question are the steps that turn two mismatched sets of numbers into a conclusion you can trust. Scientists do exactly this when results from different labs disagree: they compare methods, find the source of error, and design a cleaner test.
Quick self-check
1. Two groups tested how fast iron nails rust in water, salty water and dry air, and got different amounts of rust. Why compare their methods, not only their final amounts of rust?
2. Group A dried each nail before weighing the rust; Group B weighed the nails while still wet. The two sets of rust amounts differ. What is the most likely source of error?
3. A source of error in this rust investigation is best described as something that...
4. After comparing the two groups, which is a good question for further investigation?
5. You are selecting evidence to draw a reasoned conclusion about which condition rusts nails fastest. Which piece should you set aside?