Putting It All Together
You now know how to classify a statement as data, a conclusion, or an argument. You know how to build conclusions up from data using logical rules. You know that arguments presented without data or improperly formatted arguments are of no value unless they can be restated. And you know that your data and conclusions must all fit together like a puzzle or a well-oiled machine. When new pieces arrive, they must either fit into the puzzle (possibly by discarding old pieces) or be discarded themselves. The decision of which to discard is based on the merit of each piece, not on which one was in your mind first.
So how do you put this into action? Although the obvious path is to start sifting through your own mind and critically analyzing what is already there, I would instead recommend first getting some practice by analyzing external information for which you have no or little current knowledge. The emotional challenge of tackling what already exists in your head can cloud your reason, so it's better to start by honing your skills on information unlikely to evoke such emotion. So, pick up a newspaper, or go to a news website, and read an article at random. Complete the following checklist:
Here's a made-up example of a news clipping. Run down the checklist and see what you come up with, then compare your results with the listing below.
"An asteroid is on a collision course with Earth, say scientists. Dubbed AB175, the space rock has been measured at 30 kilometers wide and may weigh up to 1 million kilograms. 'Our calculations show that its trajectory will put it within 100,000 kilometers of the Earth in March of 2012,' says Dr. John Smith of the Westville Observatory. Dr. Joe Williams of Easttown University says, 'An asteroid like this one wiped out the dinosaurs millions of years ago. Humanity could be next if we don't do something about it.'"
Run through the checklist, and write down your answers. When you're done, compare your results to those shown below.
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Arguments:
Provided Data:
Although it starts the vague identification of "scientists," later it quotes someone who works at an observatory who refers to their calculations. This implies that the primary source is Dr. Smith and his colleagues.
The second quote is more dubious. Even though it is someone who is a doctor and is associated with a university, there is no indication that this person produced any of the data or has even examined it closely. Their field of study may not even have anything to do with astronomy - just because someone is a doctor does not make them an expert on all subjects.
Does Westville Observatory have a web page? If so, they may have posted a press release or an article of their own, with more details on their studies and calculations. For example, the article claims that the asteroid weighs "up to" a million kilograms - this may mean that the scientists estimated between half a million and one million kilos. News articles tend to take the more extreme estimates in order to make the news more exciting.
Have other observatories witnessed the asteroid, and made similar calculations? A web search for the asteroid name might locate this information. If only one group of scientists have come to this conclusion, then it is not as solid as it would be if other groups had independently verified it themselves. As well-intentioned as the original group might be, scientists are human and can make mistakes.
Was the asteroid comparable to the one which caused the extinction of the dinosaurs, as they say? Although there is much debate as to the size of any such asteroid (or whether it was the cause of the extinction at all), research might turn up estimates for the ancient asteroid's size at 100m in diameter and weight at 200,000 kilograms. So the one on its way toward Earth now is a bit smaller, but still comparable in size. The statement that its impact will cause serious damage to life on Earth is not without merit.
One consistency check that suggests itself is whether the asteroid's size make sense for its weight. One notices when comparing against the estimated size of the ancient asteroid above, that this one is much smaller in diameter, but the article lists it as being much heavier. Although not impossible, this seems a bit odd. Since the size of the asteroid is easier to guess than the weight (we can see it, but we can't weigh it), it seems likely that the size is correct and the weight is overestimated.
An obvious point of context is: how often do asteroids of this size of larger pass this closely to Earth? It may be that, looking back through past news articles on the same subject, this sorts of collision predicts are announced several times a year. If this many asteroids are in danger of striking Earth, but (to date) life on Earth has only faced serious danger from them on several occasions over the past few hundred million years, then the chances that this one are anything special seem low.
Similar to the point above, what has happened when asteroids of this size have collided with the Earth? Research might show that asteroids lose quite a lot of their weight and velocity while entering Earth's atmosphere, and therefore pose less danger when they strike. Moreover, if you can find a specific instance of an asteroid of comparable size striking in recorded history, then you can examine the effects of that collision. For example, if it struck far out in the ocean and did little except produce some minor wave disturbances at distant shores, then this would cause you to consider that asteroid strikes may only have ill effects if they strike near a populated area.
The article implies that concern is warranted because the asteroid will pass within 100,000 kilometers of the Earth. That sounds like plenty of room. Will it really be a close call? For this you need some additional data, such as the radius of the Earth, which is 6,000 kilometers. 100,000 and 6,000 are close enough for us to say that, yes, that sounds like there is a reasonable chance for the asteroid to collide with Earth. In this case a visual representation may help. Draw a circle that is 10 centimeters in radius, then another that is 0.6 centimeters in radius, both with the same centerpoint. (This is the same size radio as 100,000 and 6,000.) The inner one is the Earth; the other one is the area where the asteroid may pass. Looking at the picture may give you a better sense of the distance involved, and help you imagine the chance for a collision.
The first conclusion, that the asteroid may strike us, seems solid enough assuming you can verify that the scientists have actually made the observation of the asteroid's size and trajectory, that other scientists have seen the same thing, and that both have run calculations to determine the collision possibility and have gotten the same result.
The second conclusion is much more dubious. Although the asteroid will pass near Earth, its chances of striking it are still fairly low (look back at the picture you drew). If it does strike, its size is much less than that which caused the Cretaceous extinction. For that matter, the extinction being caused by an asteroid strike is still up for debate, so drawing comparisons to it is a bit shaky. Nevertheless, there are many parallels, so this conclusion does have some value.