ACT® · Science: Interpretation of Data (13-17 items)
Units, Axes, Legends and Keys (IOD 202)
Every graph and table on the exam is only as trustworthy as its labels: units tell you what number means, axes tell you how it climbs, and legends tell you which curve belongs to which experiment.
Before you read a single data point, read the labels. A y-axis marked mg/L and one marked g are answering different questions, even if the numbers look the same, because the unit sets the size of one unit of "stuff." Mixing them up means your answer is off by a factor of a thousand even though you read the graph perfectly. Axes also don't have to climb in equal steps. Many science passages use logarithmic axes, where labelled ticks go 1, 10, 100, 1000: each mark is ten times the one before it, not ten more. A point halfway between the 10 and 100 marks on a line like that is not 55. It sits at roughly 31.6, because the halfway point in distance is a halfway point in exponent, not in value.
Legends and keys do a different job: they tell you which curve, bar, or column belongs to which experiment, trial, or site when several are drawn on the same axes. Without the legend, three lines on one grid are just three anonymous squiggles. Check the legend every time a figure shows more than one data series, and recheck it if a passage has multiple figures using different symbols for the same substance.
This topic feeds directly into reading trends and interpolating or extrapolating values, because every one of those skills depends on first knowing what the axis actually measures. A graph that looks steep can be the slower change if its axis tops out at a bigger number than the one next to it. Always compare the actual values the axis reaches, not how the line looks on the page.
Key terms
- Logarithmic axis
- An axis where each labelled tick is ten times the value of the tick before it, not a fixed amount more.
- Legend
- A key that matches each line, bar, or symbol in a figure to the experiment or substance it represents.
- Concentration
- An amount of substance per unit of volume, such as milligrams per liter, that must be multiplied by volume to get total mass.
- Axis scale
- The range and spacing of values an axis covers, which determines how steep or flat a curve appears on the page.
Watch it explained
- How it worksHow does a legend turn three anonymous curves on one pair of axes into three separate experiments?
- How it worksWalk me through reading an axis whose ticks jump 1, 10, 100, 1000 instead of climbing by equal steps.
- Worked exampleFind the true value of a point sitting halfway between the 10 and 100 marks on a logarithmic axis.
- Common mistakeWhy doesn't a steeper-looking curve always mean faster change when two graphs use different scales?
Check yourself
End of topic.