Bar graphs: compare

Example 1

1. First, read the edges. The title: the five states with the most people, in 1900. The unit under the axis: people, in millions. The bars have no numbers on them, so the axis is the ruler.
2. Read what one unit is worth. The labels are 0, 2, 4, 6, 8, and there is a tick between each pair, so each tick is 1 million people.
3. New York’s bar ends a little past the 7 tick: about 7.3 million. Missouri’s bar ends a little past the 3 tick: about 3.1 million.
4. “How many times as many” asks you to divide, not subtract. 7.3 ÷ 3.1 is a little more than 2 (2 × 3.1 = 6.2, and 3 × 3.1 = 9.3 is too much).
5. So New York had about twice as many people as Missouri. That is B. On a bar graph you can also see it: New York’s bar is about two Missouri bars long.
6. A and C come from guessing without the axis, or from subtracting (7.3 − 3.1 = 4.2) and calling the difference “times.” A difference of about 4 million is not “4 times as many.”
7. D is wrong at a glance: the New York bar is clearly the longest, and the Missouri bar the shortest.

Answer: B

Example 2

1. Read the edges: the title (members of three locals) and the unit under the axis: members, in thousands.
2. Read what one unit is worth. The labels are 0, 10, 20, 30, 40, with a tick between each, so each tick is 5 thousand members.
3. Local A’s bar ends at the tick halfway between 30 and 40: 35 thousand. Local C’s bar ends at 10: 10 thousand.
4. “How many more” asks for the difference: 35 − 10 = 25 thousand.
5. Put the unit back: 25 thousand is 25,000 members. That is B.
6. A forgets the unit: 25 is the difference read straight off the axis, but the axis is in thousands.
7. C is Local A’s own number, not how many more. D adds the two bars (35 + 10 = 45 thousand) instead of subtracting.

Answer: B