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Command of evidence: textual (scientific) — Worked example

Learn the best way to approach a scientific command of textual evidence question on your SAT. First, spot the claim you need to support. This often the "hypothesis" in scientific texts. Then, rephrase the claim in your own words, and look for the best evidence to back it up in the choices.  Created by David Rheinstrom.

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Video transcript

- [Instructor] Hey, test takers. Let's take a look at this question from the reading and writing test. The pH level of lake water, which indicates how acidic or basic the water is, can be influenced by a variety of factors, including acid rain, rock and mineral erosion, and air pollution. Researchers in Alabama hypothesized that recent fluctuations in the pH of the water in Guntersville Lake are attributable to another source. Agricultural chemicals that have been entering the lake through runoff. Which finding, if true, would most directly support the researcher's hypothesis? If you'd like to give this one a try on your own before I teach you some specific strategies for this question type, please feel free to pause the video now. Okay, this is a science flavored command of evidence question. And these questions introduce some sort of argument or claim. Your job is to identify that argument and find the evidence that most strongly supports it. For science passages, you won't need to lean on anything you've studied previously. Everything we need to know is contained right here in the passage. We want to figure out what the hypothesis is, what the argument is, and support it. Follow me to the strategy corner, won't you please? The question will introduce a central claim or argument and it'll be stated very clearly so you won't have to go digging for it, but that's your first job. Identify the claim. Once you do that, create a test phrase that restates the claim in your own words. Doing this will give you control of the idea and allow you to see it restated differently. Once you've got your own words version of the claim test it against the choices. Whichever choice matches your test phrase will be the answer. Let's reacquaint ourselves with what the question wants us to do. Okay, so which finding if true would most directly support the researcher's hypothesis? Well, okay, look here at the second sentence. Researchers in Alabama hypothesized, there it is. That recent fluctuations in the pH of the water in Guntersville Lake are attributable to another source, agricultural chemicals that have been entering the lake through runoff, right? There it is. That's our hypothesis. pH fluctuations were caused by runoff. So let me state that another way. Our test phrase here will be runoff leads to pH change. And instead of the word change, I'm just gonna use a delta. Let's test that against the choices. Okay, so which finding would most directly support our hypothesis that runoff accounts for the change in pH? Choice A, the pH levels of otherwise similar lakes in Alabama that do not receive as much runoff from farmland as Guntersville Lake does fluctuated less than the pH of Guntersville Lake did in the same period. Hey, how about that? This matches my test. We're talking about fluctuation, right? The degree of change. And this matches my prediction or very nearly less farm runoff in similar lakes led to less fluctuation in pH. And we need to compare apples to apples here. We need to compare Guntersville Lake to other similar lakes so we can properly understand the impact of agricultural runoff. This has to be the answer on test A, I would circle it and move on. But let's knock out the other three just to say we did to prove why A is the answer. So let's keep going. Choice B, the pH levels of other lakes in Alabama that are near Guntersville Lake tended to be lower than that of Guntersville Lake regardless of the degree of fluctuation in the pH of Guntersville Lake. This doesn't support the hypothesis. The argument is not about how low the pH is, it's about how much the pH changes the fluctuation. Choice C, the amount of agricultural runoff entering Guntersville Lake steadily decreased during the period in which the lake's pH level fluctuated, while a composition of that runoff remained largely unchanged. Again, this doesn't support the hypothesis. It's not comparing Guntersville to other lakes, and it talks about the decreasing rate of runoff which isn't really relevant to the hypothesis. Choice D, both air pollution and agricultural runoff in the vicinity of Guntersville Lake increased during the period in which the pH of Guntersville Lake fluctuated. If these polluting inputs both increased and the pH level in the lake fluctuated, then this would actually weaken the hypothesis and that's not what we want. So goodbye choice D. See, once you've got that test phrase that support for the hypothesis, you can just rock it through the choices. Let's talk through a couple of top tips for questions like these. Top tip number one, be specific. You are looking for an answer choice that supports the claim in the question stem. Nothing more, nothing less. So any choices that introduce a new idea can be eliminated. We don't need evidence for anything else. Top tip number two, be strict. Anything that feels like it's almost evidence for the claim is going to be too weak to be the answer. The answer needs to be all right, not just partly right. Good luck out there test takers. You've got this.