#21 - Most large nurseries sell raspberry plants primarily

[reticulargirl]

Let me revise:

if Johnson is NOT a commercial grower (satisfying one sufficient condition of the contrapostive), then Wally's MAY not be (less than 50%) a large nursery.

Is the error in (C) the answer's use of "probably"?

For my contrapositive, I went against the "most" train to get to "some."In (c), the answer is Wally is LIKELY (not MAY) to NOT be a large nursery.

[James Finch]

Hi RG,

This stimulus gives us two pieces of information about most large nurseries:

1. Most large nurseries sell raspberry plants primarily to commercial raspberry growers, and;

2. Most large nurseries sell only plants guaranteed to be disease free.

These two pieces of information require two separate diagrams:

1. MLN CRG

and

2. MLN GDFRP, with a contrapositive of GDFRP MLN

We're then told that Johnson receives raspberry plants from Wally's that are diseased. The problem here is that we don't know anything about guarantees or whether Wally's is a large nursery or not, so we can't simply plug the information in the stimulus into our diagrams and come out with an answer. Instead, the answer choices will fill in that information for us and either include or exclude Johnson and Wally's from the categories we have diagrammed.

Answer choice (C) is asking test takers to draw an unsupportable inference: making a contrapositive relationship from a formal logic relationship including two "most" statements. We can't logically do this, however, because the contrapositive requires the presence of a true conditional relationship in which when the sufficient condition is satisfied, the necessary condition must also be satisfied, 100% of the time. The problem here is that the "most of X selling primarily to Y" setup leaves a variance too large to say anything about what happens with Y: if 50% of large nurseries are selling 50% of their stock of raspberry plants to commercial raspberry growers, that only accounts for 25% of the total stock of large nurseries' raspberry plants. On the other extreme, it's possible that 100% of large nurseries are selling 100% of their raspberry plants to commercial growers, meaning all large nurseries' raspberry plants are going to the commercial growers. But the wide range of possibilities (~75%) means we can't definitively say whether, if Johnson is not a commercial grower, that Wally's is not likely (<50%) to be a large nursery.

Answer choice (E) works because it deals with the conditional relationship given between large nurseries and selling only plants guaranteed to be disease-free. We know that if Wally's is a large nursery, it is more likely than not (as most do this) to guarantee that all the plants it sells are disease free. This means that Johnson's plants would more likely than have been guaranteed disease free, and that is likely they didn't arrive as guaranteed.

Hope this clears things up!

[reticulargirl]

Thanks, James.

My error was that I combined the two diagrams into one. I understand the problem better now.

-Sandi

[PianolessPianist]

Can someone please explain (again) why (c) is incorrect?

Here is my diagram:

Large nurseries sell --(most)--> raspberry plants to commercial growers AND sell only disease-free plants

Contrapositive: NOT commercial grower or NOT sell disease-free plants --(some)--> NOT large nursery that sells

Therefore, under (c): Johnson NOT commerclal grower, then Wally's is likely not a large commercial nursery.

What am I doing wrong here?

Hi!

Does anyone have an answer for this? I also used the same pattern of reasoning to arrive at answer C. My LSAT is in 5 days so any insights would be greatly appreciated.
thanks!

[Adam Tyson]

The confusion here is over the impact of "commercial growers." When we see that large nurseries sell primarily to commercial growers, that is telling us something about large nurseries. It is NOT telling us much, if anything, about commercial growers. All we can be sure of is that at least SOME commercial growers get their raspberry plants from large nurseries.

The negation of that term is not "if Johnson is not a commercial grower." It's "If a nursery doesn't sell primarily to commercial growers", and that would prove "then that nursery is probably not large."

Look at it another way, with numbers. Assume a large nursery sells 100 such plants. At least 51 of them go to commercial growers - most of them, or "primarily" them. But that still leaves 49 plants that could go to non-commercial folks, and Johnson could easily be one of those folks. In fact, it is possible that ALL non-commercial customers get their plants from large nurseries, as long as they typically make up less than half of all of the business. Knowing that Johnson is not commercial would, in that case, PROVE that Wally's is a large nursery!

I hope that helped!