Whole to part, part to whole: the LSAT's composition flaws

Two mirror-image errors, and the test uses both. Composition moves from the parts to the whole: every member has a property, therefore the group does. Division moves the other way: the group has a property, therefore each member does.

Neither is automatically wrong, which is what makes them harder than they look. Some properties do transfer and some do not, and the argument almost never tells you which kind it is dealing with.

The counterexample test

Try to build a case where the premise holds and the conclusion fails. It is usually quick.

• Every player on the team is excellent, so the team is excellent. A team of brilliant individuals who cannot play together. The premise holds, the conclusion fails.
• The company is enormously profitable, so each of its divisions is profitable. One division can be carrying four that are not.
• Each part of this machine is lightweight, so the machine is lightweight. Ten thousand lightweight parts.

Then compare with a property that does transfer: every brick in this wall is red, so the wall is red. Nothing is wrong with that. The difference is whether the property depends on how the parts are arranged or combined — and that is the question to ask.

Where it hides

Averages and rates are the most common disguise on this test, because they are group properties that say very little about individuals.

• The average salary rose, therefore this employee earns more. An average can rise because low earners left.
• This region has the highest rate of X, therefore this person from the region is likely to X. A rate is about the group.
• The class scored well overall, therefore each student did.

Whenever a stimulus gives you a statistic about a population and the conclusion is about a particular case, or vice versa, this is the family you are in — and it overlaps heavily with the numbers-and-percentages family, which is worth reading alongside this.

How answers describe it

• infers that what is true of each member is true of the group as a whole
• assumes without warrant that a characteristic of a group is shared by its individual members
• draws a conclusion about a particular case from a generalization about a population

On Weaken questions the attack is a counterexample of the kind you built. On Necessary Assumption questions the answer is the modest claim that the property does transfer here — and it usually reads as underwhelming, which is exactly why people pass over it.