Common Logical Flaws – Part 3
March 15, 2013
Are you a master of the flaw? When you see a commercial on TV, can you spot the advertiser’s twisty logic? If not, don’t despair – part 3 of our series on LSAT logical flaws completes our journey down Illogic Lane.
Scope Shifts: “scope shift” is just a fancy way to say that the evidence and conclusion talk about different things. You’ve learned to focus on an author’s scope in both logical reasoning and reading comprehension – after all, the most common wrong answer trap in both sections is “outside the scope”, i.e. answers that are irrelevant to the question.
In logical flaw stimuli, you should always be on the looking for terms in the conclusion which don’t match up with the evidence. Here’s an example:
Advertisement: The XJ2000 is the most technologically advanced washing machine on the market. If you want to avoid costly repairs, therefore, you should buy an XJ2000.
Did you spot the shift in focus? The evidence is that the XJ2000 is technologically advanced; the conclusion, however, is about durability and reliability – which is completely different! I mean, if you actually think about it, the least advanced washing machine – a river, a rock and a big stick – is the one that requires the least repairs.
A common variation of the scope shift is a questionable analogy; the evidence is a story about something irrelevant to the subject matter of the conclusion.
Here’s our first takeaway: always compare the substance of the evidence and the conclusion – if they’re different, you’ve identified a scope shift.
Numbers vs Percents: “numbers vs percents” is a specific type of scope shift that appears frequently enough to deserve its own sub-category. In these arguments, either the evidence is about hard numbers and the conclusion is about percents, probabilities or rates or vice-versa. Here’s an example of each:
Every year, millions of people have accidents while cooking breakfast, yet only a few hundred have mishaps while skydiving. Clearly, it’s safer to jump out of plane than to make eggs.
Can you see what’s wrong with this reasoning? The evidence is about the number of injuries, but the conclusion is about safety – a probability or rate. What’s involved in calculating safety? Not just the number of injuries, but also the number of participants. What’s an alternative explanation for the greater number of breakfast injuries? Every year billions of breakfasts are cooked; far far fewer people go skydiving!
The tax rate is higher in Rhode Island than in California. It should be obvious, therefore, that Rhode Island collects more tax than does California.
Again, we have one part rate and one part numbers – just reversed from our last example. What has the author overlooked that makes us seriously doubt the conclusion? There are a lot more taxpayers in California than there are in Rhode Island!
Time for our next takeaway: when one component of the argument is numbers and the other is rates/percents/probability, the author is always overlooking the number of objects in each group.
Possibility vs Certainty: an LSAT grandmaster is always on the lookout for qualifying language – many LSAT points have been won by focusing on limiting terms such as “some”, “most” and “all”. A discerning eye will also be your friend on logical flaw questions.
A possibility vs certainly flaw involves a shift in such language – almost always going from speculative evidence to a confident conclusion. For instance:
The Surgeon General just released a report the definitively shows that smoking can cause lung cancer. My cousin Bob smokes. Sadly, unless Bob quits smoking he’s definitely going to get lung cancer.
Did your soon-to-be-well-trained eye spot the shift in language? The report shows that smoking can cause lung cancer, but the author concludes that Bob will get lung cancer. That’s a pretty big jump!
Our final flaw takeaway: always be on the lookout for qualifying language; if the language is inconsistent, then identify the flaw of possibility vs certainty.
Whew! Six common flaws under your belt – you’re well on your way to mastering LSAT flaw questions. The better you get at quickly identifying these patterns, as well as the others that appear throughout the LSAT, the quicker you’ll be racking up points on LSAT test day.
Here’s some fun homework – next time you see a commercial on TV or hear one on the radio, see if you can spot a logical flaw – there are plenty of them there as well! Then share those flaws with our community.