Help me understand why this is a BAD argument? Forum

[LSAT_Padawan]

It is clear that most of Janine's friends are good drivers, since she accepts rides only from good drivers and she accepts rides from most of her friends.

I diagrammed it this way:

Janine's friends: JF
good drivers: GD

P: GD
P: JF
C: JF most -> GD

Where did I go wrong? Thank you for your enlightenment.

[3ThrowAway99]

I think that the logic here is sound unless further conditions were given excepting her friend group from the necessity of a person being a good driver for her to ride with them. At first I thought the argument was flawed because it seemed to me that the "most" applied to different sets (the portion of her friends who are good drivers and an overlapping but possibly distinct group including a general majority of her friends). In any case, even if a group of 'most' of her friends can be different than 'most' of her friends who are good drivers it doesn't affect this argument because the conclusion simply requires sufficiency that a majority is met (which the combination of the premises shows us that this sufficiency is the case) and not a necessity that that particular majority situation is consistent with all majority scenarios. I think the argument is sound. I think your diagramming is not accurately reflective though. I am not very good at diagramming arguments so perhaps someone else could offer suggestions in terms of the diagramming.

I perhaps would write it like this, but I'm not sure if this is at all helpful:


1)Janine only accepts rides from good drivers: JGD

+

2)Janine accepts rides from most of her friends: JMF

=

3)Most of Janine's friends are good drivers MJFGD

P: JGD
P: JMF
C: JGD + JMF -> MJFGD
(contrapositive being -MJFGD -> -JGD or -JMF)

[kyle]

the class of friends as a whole is potentially much larger than the class of friends she accepts rides from.


i cant think of any other reason it might be bad

[BigTabacco]

There is a consideration on how many of Janine's friends are drivers at all.

If she has 100 friends, 10 of which can drive, and she will ride with 9 of them, only 9% of her friends are good drivers.

Hence it doesn't have to be true that most of her friends are good drivers, only "most of her friends that can drive" are good drivers.

It is unreasonable to assume that being a friend and being able to drive are interchangeable.

[tomhobbes]

The argument seems perfectly fine to me, unless you interpret one of the premises in a slightly weird way.

1. Janine accepts rides only from good drivers.

Therefore, if Janine accepts a ride from someone, they are a good driver.

2. Janine accepts rides from most of her friends.

There are two valid interpretations of this, so this might be the problem. It could either mean literally what it says, which is that out of the total group of her friends, she accepts rides from most members of that group, or it could mean that out of the members of the group who offer her rides, she'll accept that offer from most of them. Does that make sense?

In the first sense, this would be a good argument. Under the second sense, where Premise 2 translates to "Out of the pool of Janine's friends who offer her rides, she will accept rides from most of them," it would be a bad argument. In the second sense, the pool of friends who offer her rides could be a small subset of the total group of her friends.

I hope that made sense. Where is this argument from?

[KG_CalGuy]

There is a consideration on how many of Janine's friends are drivers at all.

If she has 100 friends, 10 of which can drive, and she will ride with 9 of them, only 9% of her friends are good drivers.

Hence it doesn't have to be true that most of her friends are good drivers, only "most of her friends that can drive" are good drivers.

It is unreasonable to assume that being a friend and being able to drive are interchangeable.

This is also the only reason I can think of. The question seems designed to make you equate friends with drivers. She may accept rides from most of her friends (that offer), but most of her friends may not be drivers. On LSAT it's probably either an numbers or an assumption flaw.

[3ThrowAway99]

I would agree with the poster who indicated the argument is not sound based on the idea that not all her friends necessarily drive if the second premise read "she accepts rides from most of her friends who drive" rather than what it does say which is "she accepts rides from most of her friends". Because the latter is the case I maintain that the argument is valid and that the proportion scenario does not alter the validity.

This basically echos what tomhobbes stated regarding the argument.

[gatorgirl4life]

it isn't a bad argument. since "only" precedes a necessary condition, you could say that
Janine accepts ride (sufficient) --> good driver (necessary).
Janine accepts ride from most friends --> most friends are good drivers

more importantly, this exact argument appeared in my lesson #5 homework for testmasters, and the answer key says that it's a good argument.