Inferences on Logic Games

    • April 23, 2016 at 3:40 pm #1771
      LSAT Dan

      On some games, there really aren’t many (if any) available inferences up front, and time you spend looking for them is basically time wasted, when you should have been hitting the questions.  On the other hand, on some games, there are some really huge inferences available, and if you rush to the questions, you’re going to miss something that could have saved you a lot of time and effort.  So what do you do?  It’s not really helpful to say, “Sometimes it’s a good idea to look for inferences and other times it’s not.”

      Here’s the first thing you should know:

      Q: Where do inferences come from?

      A: They come from those times when the same variable is affected by more than one rule.

      That’s the basis of inferences.  For instance, take Preptest 62 (December, 2010), game #2 – Here are two selected rules:

      “If a window does not contain purple glass, then that window contains orange glass.”  Anytime you have a conditional rule on a logic game, you want to add the contrapositive to your checklist.  The rule is:

      ~P –> O.   The contrapositive is:

      ~O –> P

      “If a window contains yellow glass, then that window contains neither green glass nor orange glass.”

      Each of these rules involves orange, and sure enough, they give us a basis for an inference: If a window contains yellow glass, then it must also contain purple glass.  Because if it has yellow, it doesn’t have orange, and if it doesn’t have orange, then it has purple.

      That’s not really an earth-shattering or difficult example; I just wanted a handy real life example to illustrate the point.  Inferences come from multiple rules affecting the same variable.

      Corollary: If a game doesn’t have multiple rules affecting the same variable, it’s very unlikely (albeit not impossible) that you’re going to find much in the way of inferences.

      Additionally: Some rules are “either/or” rules, and those rules generally don’t provide the basis for much help finding inferences.  For instance, Preptest 66 (June 2012):

      “Either Parra or Tiao (but not both) works in Zone 1.”

      “Parra and Quinn work in the same sales zone as each other.”

      Here, we have the same variable, P, mentioned in a couple of rules.  But that first rule is an either/or rule, so it’s not going to give us anything solid to draw inferences from.  It tells us something about Zone 1 (there’s at least one person there), but what does it really tell us about P?  Well, it tells us that he might work in Zone 1…or he might not.  Combining that with the P/Q rule, we learn…absolutely nothing about Q.  So, in imageless flowchart style:

      1.  Are there multiple rules affecting the same variable?  If not, go to 5, below.  If so, go to 2, below.
      2. Do the rules give you something definite about the variables they affect?  If not, go to 5, below.  If so, go to 3, below.
      3. Focus on the variables affected by multiple rules; work out what you can from them, then go to 4, below.
      4. Are there any variables affected by initial rules and also by something you figured out under Step 3, above?  If so, go to 3, above, and focus this time on those variables.  If not, go to 5, below.
      5. Get to the questions!


    • April 24, 2016 at 2:38 am #1772

      Thank you for that, Dan.

      I am following your steps and it seems to have helped a bit so far.

      I continue to chug along with these games!

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