Two 4th classes, Rang Rua and Rang Gorm, chose their favourite lunch. Their choices are on one chart. Each lunch has two coloured bars: one for Rang Rua and one for Rang Gorm. The key tells you which colour is which class.

Just look and notice: what is the same about the two classes, and what is different? There is no wrong answer here. If you are not sure whether one lunch is really more popular in one class than the other, that is fine. We will learn how to check that fairly in a moment.

Here is that same chart again. Each lunch still has two coloured bars, one for Rang Rua and one for Rang Gorm, and the key tells you which colour is which class.

The scale is the row of numbers going up the side. It tells you how tall each bar is. This chart goes up in even steps to 15, and both classes share that one scale.

Before anyone compares heights, check the scale is the same for both bars. Here is why that matters, using sandwich. Rang Rua is on 12 and Rang Gorm is on 9. On this shared scale, Rua's bar is taller, so sandwich really was chosen more in Rang Rua.

Now picture an unfair drawing of the same numbers: Rang Rua still on a scale to 15, but Rang Gorm drawn on a scale that only goes up to 9. Gorm's bar of 9 would fill its whole side and look taller than Rua's 12, even though 9 is less than 12. The heights would trick you. That is why two bars must use the same scale before you compare them.

With our fair shared scale locked in, watch for the lunches that were higher in one class, the lunches that were lower, and the lunch that barely differed.

Today we build the comparison ourselves. Here are two weeks of the school library: how many books were borrowed each day.

We will build Week 1 first, then add Week 2 one day at a time. Before each Week 2 bar goes up, say whether that day rose, fell, or stayed the same.

Today we read two data sets together and back up every answer with the chart. On the board are two teams' scores across four rounds of a class quiz game.

We'll work through these together. Point to the bars you used before you answer.

• Which team was ahead in Round 1?
• Which team was ahead in Round 4?
• Which round were the teams level?
• Which team improved most from Round 1 to Round 4?