Grades 9–12 · History & Evidence
Did the canal cause a port to grow?
A change that follows an event is not automatically caused by that event. A historical causal explanation needs more than a before-and-after sequence: it must address other changes that could help account for the result.
In the diagramDid a canal cause the growth?Fictional ports; A opens its canal in 1850Port APort BCanal + rail linkNo new canal10018481601852140Annual shipments in the same unitGrowth: A +60; B +40. Cause still uncertain.
In the fictional archive, Port A opens a canal in 1850. Annual shipments increase from 100 in 1848 to 160 in 1852, a rise of 60 in the stated shipment unit. The canal comes before the later total, so the order is compatible with an effect. The totals alone do not isolate that effect.
Port B provides a comparison: it has no new canal, yet shipments rise from 100 to 140. Growth in both ports could reflect a wider regional change. A’s increase exceeds B’s by 20, but that descriptive difference is not automatically the canal’s contribution. B has not been established as a picture of what would have happened to A without its canal.
A further fictional record shows that A also gained a rail link in 1850. This competing change makes a canal-only explanation even less secure. Comparable records, routes and other relevant developments can help distinguish explanations, but the past cannot simply be rerun with one feature removed. The main implication is that time order and comparison are useful evidence, not substitutes for the assumptions and supporting records needed for a causal claim.
About this explanation
Starting knowledge: Calculate differences, read paired comparisons and distinguish a causal claim from an observed association.
Sources
- National Park Service — Historic Places and the Inquiry Method
- OpenStax — Scatter Plots, Correlation, and Regression Lines