About this video
This LSAT-style lesson works through a necessary-assumption question. A researcher recaptured tagged armadillos and found most were near their tagging site, concluding they do not move rapidly into new territories. The key assumption is that the recaptured armadillos did not travel to a new territory and return during the intervening summer.
You'll be able to
- identify the conclusion and evidence in an argument about animal movement.
- test answer choices by asking whether the argument would fail without them.
- distinguish a necessary assumption from a merely supporting or irrelevant one.
Key words
- Necessary assumption
- A claim that must be true for the argument's conclusion to follow, so negating it would weaken the argument.
- Recapture
- Finding again animals that were tagged earlier, which is the evidence used here.
- Intervening period
- The time between two observations, here the summer between the spring tagging and the fall recapture.
- Negation test
- Negating an answer choice and checking whether the argument then collapses, which reveals a necessary assumption.
Transcript
- [Instructor] So a researcher is saying this fall I returned to a research site to recover the armadillos I had tagged there the previous spring. Let me just write this. So previous spring. Previous, previous spring. Then there is this fall, this fall. So we're moving in time and he returns to a site to recover armadillos.
So let's say that this is a broader region and he's returning to a site, so this is the site right over here. And he returns to that site, so I'm drawing the broader region and I have this site over here. So this fall I returned to a research site to recover the armadillos I had tagged the previous spring.
So let's say here are the armadillos that he tagged the previous spring. Since a large majority of the armadillos I recaptured were found within a few hundred yards of the location of their tagging last spring. So he's recapturing them so these are ones that he tagged before.
He's saying a large majority were found within a few hundred yards of the location of their tagging last spring. He concludes that armadillos do not move rapidly into new territories. I concluded that armadillos do not move rapidly into new territories. Now before I even look at the choices, and we're supposed to answer the question, which of the following is an assumption required by the researchers argument?
One question that immediately jumps in my brain, well he's assuming that they didn't go some place in between. Maybe they went, you know, maybe they went here during the summer, which is quite far away, and then by fall they came back. So there's some type of migratory behavior.
He's assuming that in order to conclude that armadillos so not move rapidly into new territories because they might have moved super fast into another territory and then come back. So let's see if any of these choices speak to that. Of the armadillos living in the area of the tagging site last spring, few were able to avoid being tagged by the researcher.
Well no, his assumption isn't based on the proportion of armadillos, of all the armadillos he was able to tag. He's just saying that of the ones I'm recapturing, a lot of them seem to be near the same area so it doesn't seem to be a key assumption required. Most of the armadillos tagged the previous spring were not recaptured during the subsequent fall.
No, that's not a key assumption that he's making. In fact, that would undermine his position if he's only able to get a very small fraction. Most of the armadillos tagged the previous spring were not recaptured the subsequent fall. That would actually give a problem to his argument.
Predators did not kill any of the armadillos that had been tagged the previous spring. No, he's not saying that he recaptured all the armadillos or he needs to recapture all the armadillos. It's possible that predators killed a few of them but he's recapturing, he's saying that of the ones that he recaptured, that the majority were within a few hundred yards of this same location.
So it doesn't mean that he has to get all of them back, that we can't, you know, all of them have to be alive from one year to the next. It's just most of them have to be around. D: The tags identifying the armadillos cannot be removed by the armadillos, either by accident or deliberately.
Now once again, this is very close to C. He's not saying that he has to, that all the tags have to be there. As he said, he's just saying, the majority of the armadillos he recaptured, so he is assuming he had to recapture a good chunk of them, but not all of them.
So even if some of the tags get removed or some predators kill some of the armadillos, doesn't seem like that big of a deal. Now let's go to the last choice, which is hopefully the answer since I ruled out the first four. A large majority of the recaptured armadillos did not move to a new territory in the intervening summer and then move back to the old territory by the fall.
Well this is actually almost word for word what I was having trouble with before. He's assuming, or she is assuming, the researcher is assuming that they didn't go, that the large majority did not move to a new territory and then come back in the intervening summer.
So this for is sure the key assumption required by the researchers argument.
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