Avoid illogical comparisons and unclear relationships between ideas.
Compare equivalent things and make the logical relationship clear.
Core Idea
A comparison must compare equivalent things. Comparing an author's style to another author (instead of to that author's style) — or comparing a quantity from one year to a different year (instead of to that year's quantity) — creates an illogical comparison the SAT will test you on.
Understanding
The most common comparison error on the SAT: comparing two things that aren't logically equivalent.
The population of Tokyo is larger than London.
This compares a population to a city — that's not the same category. Fix: ...larger than that of London ("that" stands in for "the population").
The SAT uses three tools to fix illogical comparisons:
- "that of" / "those of" — replaces the noun to avoid repetition while keeping the comparison logical: The salary of a surgeon is higher than that of a teacher.
- "do" / "does" / "did" — replaces a verb: She runs faster than he does.
- Restating the noun — The wings of a hawk are longer than the wings of a sparrow.
Beyond comparisons, the SAT also tests for unclear relationships between ideas — sentences where the logical connection between clauses is muddled. Does the second clause contrast with the first? Result from it? Simply add to it? The grammar must make that relationship clear.
Always check: are you comparing apple to apple, or apple to orange?
Step by Step
- Identify what's being compared — underline both sides of the comparison.
- Ask: are these the same type of thing? (cost vs. cost, method vs. method, style vs. style)
- If not — look for an answer choice that uses "that of," "those of," or restates the noun to make the comparison logical.
- For unclear relationships: identify what logical connector the sentence uses (contrast, cause, addition) and make sure it matches the actual relationship between the ideas.
- Eliminate any answer that compares unequal categories or creates an illogical connection.
Misconceptions
- Missing the comparison error because the sentence sounds natural in speech. "Her grades are better than her classmates" is common in conversation but illogical on the SAT (grades vs. classmates).
- Thinking "that of" and "those of" are unnecessarily wordy. They're essential for creating logical comparisons when you can't repeat the full noun.
- Forgetting that "like" introduces noun comparisons while "as" introduces clause comparisons. "Like the previous study, this one..." (comparing studies). "As the previous study showed, this one..." (introducing a clause).
Worked Example
A study published in the Journal of Environmental Science found that the air quality index in cities with extensive public transit systems is significantly better than __________ with predominantly car-dependent infrastructure.
Select an answer to see the explanation