Compare Map Boundaries Before Comparing Totals

Supplied materials

• Original fictional River Market data: Map A defines “market district” as Blocks 1–3 and lists 12 stalls: Block 1 has 3, Block 2 has 4, Block 3 has 5. Map B defines it as Blocks 1–4 and lists 18 stalls, with the same 3,4,5 in Blocks 1–3 and 6 in Block 4. Both records are dated October 1 and count rented stalls, not occupied stalls or sales.

Learn the idea

A map boundary defines the area included in a statistic. If one map includes an extra block, a larger total may reflect broader coverage rather than growth or disagreement. Check date, boundary, and counted category before comparing totals. Geographic names can sound identical even when the underlying areas differ.

To make a like-for-like comparison here, use the shared Blocks 1–3. Both sources list twelve rented stalls there. Map B’s extra six are explicitly in Block 4, not evidence of a later increase, because both maps have the same date. Rental counts also leave attendance, sales, and stall-opening hours unmeasured.

1. List each map’s included blocks, date, and measured category.
2. Add the shared-block values 3+4+5.
3. Locate the source of the six-stall total difference.
4. Rewrite “the market grew from 12 to 18” as a boundary-aware statement.

Check your reasoning

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Take it further

Changed data: Map B still includes Blocks 1–4 but is dated November 1 and lists 4,5,6,7 rented stalls respectively. Model reasoning: “The shared area now totals 15 versus October’s 12, an increase of three rented stalls. B’s full total is 22, but comparing 22 with 12 would still mix time change with boundary change.”

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