Flaws in LSAT Logical Reasoning: Errors of Composition and Division

    LSAT Prep | LSAT Logical Reasoning

    [Read Part 1 , Part 2 , and lrb 4001Part 3 of this series here.]

    Before we continue with Part Four of our examination of common flaws found in LR questions on the LSAT, let’s once again take a brief moment to review why it’s so important to understand these argumentative errors. Here’s how I began the first post in this series, where we looked at 
    Source Arguments:

    Considering the vast majority of LSAT Logical Reasoning questions will have an argument in their stimulus, and the vast majority of those arguments will contain some sort of flawed reasoning, I thought I would take a moment to address a variety of the flaws that tend to appear with some frequency. In a series of posts I’ll examine a number of common mistakes that authors on the test make, which should prove useful for both Flaw in the Reasoning questions (a type that accounts for about 15% of all LR questions), as well as other question types that require you to respond to argumentation.

    With that in mind, let’s turn our attention to another type of mistake that LSAT authors make, and certainly one you have likely encountered in your day-to-day experiences: Errors of Composition and Division. These are actually two very closely related errors that involve questionable judgments relating an entire group to parts of the group. Taken together, you can consider these flaws a violation of this basic rule: “characteristics of an entire group are not necessarily applicable to all members of the group, and individual members of a group are not necessarily representative of the group as a whole.” However, since “Composition” and “Division” represent two separate mistakes, let’s examine them individually.

    1. An error of Composition occurs when an author attributes some characteristic of members within a group or entity to either the group or entity as a whole, or to each member of the group. Quite simply that which a group is composed of may not represent the group itself. Consider an example:

    “Gas prices have been rising steadily these past few years. Thus, the cost of car ownership is higher than it was previously.”

    Consider whether that conclusion—the cost of owning a car has risen—is truly proven by the premise that gas prices have gone up. Of course not. There are many, many factors that contribute to the overall cost of car ownership, only one of which is the cost of gas (and with electric cars becoming more popular gas may not be a factor at all). So you cannot assume that just because you know something about a component or entity within a group that the same information will apply to the group as a whole (or to the entire entity being considered).

    On the LSAT this type of error would be represented in the answer choices as:

    “assuming that because something is true of each of the parts of a whole it is true of the whole itself”

    “improperly infers that each and every member has a certain characteristic from the premise that many members have that characteristic”


    2. An error of Division commits a similar mistake, but in the opposite direction: an author believes that some truth about an entire group must also be true about each member of the group. That is, you cannot reliably presume facts about an entire group will still apply when the group itself is divided into smaller pieces. Let’s see how this might appear:

    “The United States is the wealthiest nation in the world. So every American is wealthy.”

    Again, it should be clear that the conclusion is not necessarily true! While we can make the claim that America is a wealthy nation, to assume that the same is true for every American (each is wealthy) would be a mistake.

    The test makers will describe this flaw as follows:

    “presumes, without providing justification, that what is true of a whole must also be true of its constituent parts”


    Granted, those examples are likely more simplistic than the ones you will encounter on the LSAT, but they do illustrate the underlying idea in each error and that’s what is fundamental to your ability to succeed when things get trickier. And the good news is that, just as the error is hopefully easy to recognize above, so too will it be easy to see on test day.  

    A final point about arguments of this type, and one that I have stressed previously: this is a very specific type of mistake and follows from a very consistent construction, so unless you see a presumed relationship between a group/whole to pieces of that group/whole, do NOT pick an answer like those above. While this type of answer isn’t as commonly given as a trap as some of the other types we’ve seen, it does come up incorrectly at times as the test makers try to trick you. So be wary when you inevitably encounter them.

    Keep an eye out for additional posts addressing a wide variety of other flaws you are likely to encounter on test day. Commit them all to memory and you’ll find yourself well prepared to respond to nearly any argument you come across.