Graph evidence and fair-comparison lab

This lab helps you read a graph carefully: check the units, the period and what is being counted, then work out what the numbers really say. It suits students who meet charts in comprehension passages and need to separate evidence from impression.

What is this lab for?

Two graphs of the same numbers can look very different. One starts its axis at zero and the other starts near the lowest value, which makes a small change look large.

The lab shows both side by side. You choose how the second chart is drawn, and the lab calculates what stays the same: the difference, the percentage change and the rate over time.

How do I use it?

1. Pick one of the three fictional datasets and read its unit, what is counted, the period and the question.
2. Choose where the vertical axis starts for the second chart. For time data, choose true or equal spacing.
3. Choose two observations to compare and read the calculated results.
4. Type the percentage change yourself and press Check my number.
5. Write one sentence on what the data show and one on what they cannot show, then open the reviewed interpretation.
6. Try the follow-up task and edit values to see what changes.

How does it work?

The difference is the later value minus the earlier value. The percentage change divides that difference by the earlier value and multiplies by 100.

For data over time, the lab also divides the difference by the gap between readings to give a rate. For groups of different sizes, it divides each total by the group size so you compare fair rates.

The lab refuses calculations that have no meaning. A percentage from a zero starting value is undefined, and an absent observation is never treated as zero. A baseline that is not below the lowest value is rejected with a message.

A worked example

This is original data. Week 1 has 40 books borrowed and Week 2 has 44. The difference is 4 books, and 4 divided by 40 is 0.1, so the rise is 10%.

Now crop the axis to start at 39. The bar for 44 stands 5 above the axis start and the bar for 40 stands 1 above it, so the taller bar looks 5 times as tall. The real ratio is 1.1.

The cropped chart changes how big the gap looks. It does not change the 4 books, and it does not say why borrowing rose.

What are the limits?

This is classroom evidence literacy with fictional data. It does not test real-world causes, judge policies or predict any SPM mark.

The numeric check covers percentage change only. Questions about causes depend on your own reasoning, which is why the lab asks you to write first and only then shows the reviewed interpretation.

Where does this habit show up?

Separating what the evidence shows from what you would like it to show matters in language papers too. See checking whether every paragraph answers the question and choosing a concrete example that actually supports the paragraph claim.

English readers can practise the same care in tracking references across paragraphs, and Chinese writers in separating evidence from repeated assertion. If you want a teacher to go through your reasoning, see the one-hour trial class (from RM50).