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saskar3 on November 21, 2019

Sufficient and Necessary

I just want to know when exactly to use sufficient and necessary strategies on the test, I know that the logical reasoning portions places heavy emphasis on it. But from the video it seems like I should be expecting it everywhere. My main question is how do I identify S & N and how to know when to use it.

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SamA on November 21, 2019

Hello @saskar3,

You are correct, a good understanding of sufficient and necessary reasoning is critical for both logical reasoning and logic games. To put it simply, it is the logic of if/then statements. It is easy to identify when it appears in the if/then format.

If the dolphins make this field goal (FG), then they will win the game (WG).
FG - - - - - - - -> WG
suf nec

However, it is not always so obvious. There are other words and phrases to look out for, such as "only," "unless," "no one," "without," or "when." Sometimes it helps to rephrase these sentences as an if/then statement. I'll give you some examples.

I won't go to the store unless I get my paycheck.
Rephrase: If I go to the store (GS), then I got my paycheck (P).
GS - - - - - - - - - -> P
suf nec

Emperor penguins are the only animals that mate for life.
Rephrase: If an animal is not an Emperor penguin (not EP), then it does not mate for life (not MFL).
not EP - - - - - - - - -> not MFL
suf nec

No one gets on the train without buying a ticket.
Rephrase: If someone gets on the train (GT), then they bought a ticket (BT).
GT - - - - - - - - - - - > BT
suf nec

saskar3 on November 21, 2019

That makes sense, I know how to identify S & N however when and how do I implement the knowledge about these statements to get the correct answer on logical games and logical reasoning?

BenMingov on November 28, 2019

Hi Saskar3,

How heavily one relies on diagramming conditional relationships is very dependent on the student's preferences. It is easy to get carried away by diagramming conditional statements every single time you see an indicator, but I have found this to be excessive.

I typically use my judgment to determine whether a question would benefit from my diagramming based on whether the entire stimulus hinges on conditional reasoning.

An approach that I apply myself is to diagram all Must Be True, Parallel Reasoning, and Error in Reasoning questions that are focused on conditional reasoning. You can argue that many assumption questions would benefit from diagrams, but this again comes down to personal preference.

How you make best use of this knowledge is by combing statements and finding inferences.

1) G - > Z
2) NOT X - > NOT Z

When we have more complicated examples with negated terms, the contrapositives are always helpful. This is why it is good form to always take the contrapositive when writing conditional diagrams.

This becomes:

1) G - > Z (NOT Z - > NOT G)
2) NOT X - > NOT Z (Z - > X)

At which point it should become more clear that we can link these statements and deduce that G - > X (G - > Z - > X)

Does this help? Please let me know if you have any other questions.