PrepTest 149, Section 3, Question 21
In an experiment, subjects were shown a series(84 more words not shown)
If all of the statements(12 more words not shown)
- AIf the subjects(30 more words not shown)
- BBasing one's guesses(33 more words not shown)
- CThere was no(17 more words not shown)
- DSome of the(19 more words not shown)
- EThe most rational(25 more words not shown)
Reading the argument
How did they do worse than just guessing 'top' every time?
Imagine 75 of 100 images appear at the top. Those aren't the experiment's exact numbers, but they illustrate the surprise: always guessing top would get 75 right without discovering any complicated sequence. These subjects instead got fewer than half right. Perhaps they kept thinking a bottom image was due after several top images, though we aren't told exactly what patterns they thought they saw.
What we’re looking for
They must have guessed bottom at least sometimes. Otherwise, they would have followed the successful always-top strategy. That's one thing we can establish before looking at the answers. For this must-be-true question, let's see what else the stated results force us to accept.
Working the answers
- A
Always guessing top could itself reflect a perceived pattern: 'Most images appear at the top, so I'll keep choosing top.' We don't know what reasoning subjects would have used if they had followed that strategy. A rules out a possible explanation without support.
- B
This compares following perceived patterns with guessing that a previous result will happen next. That isn't the comparison we're given. After a bottom image, repeating that result would mean guessing bottom; always guessing top would not. We also can't turn this particular experiment into a general rule about which strategy works better in other situations.
- C
Poor performance doesn't prove no predictable pattern existed. The subjects could have misunderstood a real pattern or looked for the wrong one. We know their method didn't work well, not that no reasonable pattern could have been found.
- DCorrect
We need one more step beyond showing that they guessed bottom: some of those bottom guesses must have been wrong.
Suppose every bottom guess was correct. Then whenever an image actually appeared at the top, the subject couldn't have guessed bottom—that would be a wrong bottom guess. They would have guessed top and gotten every top image right. Since top images made up more than half the total, that alone would give them more than half correct. But they got fewer than half correct. So some wrong bottom guesses must have occurred.
- E
Always guessing top would outperform what these subjects actually did. That doesn't make it the most rational possible strategy. Perhaps a real pattern would let someone do even better. The passage establishes a useful comparison, not that every alternative strategy is worse.
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