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False positives and false negatives

akistotleakistotle Member ⭐ 🍌🍌
edited January 2018 in Logical Reasoning 9382 karma

Here is a concept which LSAT sometimes tests:

false positive/false negative

I learned this concept a couple months ago and went:
https://media.giphy.com/media/wWuNgWHR7ZzMI/giphy.gif

Unlike other concepts, I don’t see it often on the LSAT, so I tend to forget about it.

But I see it again and again:
PT11.S2.Q15; PT15.S3.Q21; PT41.S3.Q17; PT45.S1.Q24; PT54.S4.Q20; PT61.S2.Q20; PT80.S1.Q10

So I'm making this post so that I can explain and understand fully!

False positive:
a result that shows something exists when it actually does not exist.

　Examples:
　・A medical test shows that someone has a disease when the person actually doesn’t.
　・A DNA test falsely shows that A’s DNA matches the DNA at a crime scene when it doesn’t actually match.

False negative:
a result that shows something does not exist when it actually exists.

　Examples:
　・A medical test shows that someone doesn't have a disease when the person actually does.
　・A DNA test falsely shows that A’s DNA does not match the DNA at a crime scene when it actually matches.

https://media.giphy.com/media/GSpDlV6BbFTyw/giphy.gif

In order for a test to be good, both the false positive rate and the false negative rate have to be low.

To sum up:
• True positive: correctly identified
• False positive: falsely identified
• True negative: correctly not identified
• False negative: falsely not identified

Actual LSAT questions:

:star: PT11.S2.Q15 Flaw
The computer security system has never incorrectly accepted someone, which means it has never committed a false positive error (falsely identified). The argument concludes that it will give access to the right people.

But what about false negative? In other words, it might not accept people who should be accepted. So this is the flaw.

:star: PT15.S3.Q21 MSS/Misc.
The EEG reading is a reasonably reliable indicator of temporal lobe epilepsy, so false positive rate is low.

But EEG test might not be able to detect abnormal electrical impulses even though the impulses are present (“false negative”). So not being identified doesn’t mean you don’t have temporal lobe epilepsy.

Let me know if there are other examples (other than the ones mentioned above)! :)

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Comments

  • LSATSurvivorLSATSurvivor Alum Member
    edited January 2018 228 karma

    Thank you so much for sharing!! This is awesome. Just wondering, is there a typo for the first example of false negative?

  • akistotleakistotle Member ⭐ 🍌🍌
    9382 karma

    @LSATSurvivor said:
    Thank you so much for sharing!! This is awesome. Just wondering, is there a typo for the first example of false negative?

    Yes, there was a typo :sweat_smile: Thank you for pointing out!!

  • Grace...Grace... Alum Member
    344 karma

    Helpful to identify with those labels. Thank you.

  • _oshun1__oshun1_ Alum Member
    3652 karma

    i'm just commenting on this to bump it up on the discussion posts bc this was really useful to me for pt54

  • akistotleakistotle Member ⭐ 🍌🍌
    9382 karma

    @"surfy surf" said:
    i'm just commenting on this to bump it up on the discussion posts bc this was really useful to me for pt54

    Glad it was helpful! :smiley:

  • keets993keets993 Alum Member ⭐ ⏰ 🍌
    6050 karma

    Bump for how useful this is and should probably be included in the curriculum. We need a "misc" section of concepts that aren't super frequent but frequent enough, like the tethering assumption.

  • akistotleakistotle Member ⭐ 🍌🍌
    edited August 2018 9382 karma

    This is related to the concept of "overinclusive" and "underinclusive" in law.

    • Overinclusive laws catch more cases that necessary. When someone gets caught under a law when it wasn't the law's intent to catch her/him, the law is overinclusive. That can be considered as a false positive.

    • Underinclusive laws don't catch cases which the laws meant to catch (because of loopholes etc.) You can think of this as a false negative. When a law is too specific, it tends to be underinclusive.

    @"J.Y. Ping" uses this concept in substitution/equivalence questions on the LG.

    The ideal set being a subset of your law/test set = overinclusive = false positive
    https://imgur.com/ueZ4AoF

    The ideal set being a superset of your law/test set = underinclusive = false negative
    https://imgur.com/Wy9v7L9

  • samantha.ashley92samantha.ashley92 Alum Member
    edited August 2018 1777 karma

    I think that because of my interest in medicine and general knowledge of the subject, I never really saw this as a theme. Now that I think about it, I think that this could be a really tough concept to understand-- especially for ESL students. What I have noticed as a theme is the correlation/causation concept in medical-themed questions. I don't know if I'd go as far as to say that as a general rule these questions are going to be linked to a correlation/causation answer choice, but that has been my experience in the medical questions I've seen.

  • GreatDay8GreatDay8 Alum Member
    130 karma

    @keets993 what's the tethering assumption?

  • akistotleakistotle Member ⭐ 🍌🍌
    9382 karma

    @GreatDay8 said:
    @keets993 what's the tethering assumption?

    @BinghamtonDave's thread: "Rare argument form: the "Tether" assumption"
    https://7sage.com/discussion/#/discussion/16920/rare-argument-form-the-tether-assumption

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