LSAT 149 – Section 3 – Question 21

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PT149 S3 Q21
+LR
Must be true +MBT
Quantifier +Quant
Math +Math
A
24%
161
B
5%
156
C
6%
157
D
55%
166
E
10%
157
152
161
170
+Hardest 147.456 +SubsectionMedium

In an experiment, subjects were shown a series of images on a computer screen, appearing usually at the top but occasionally at the bottom. Subjects were asked to guess each time where the next image would appear on the screen. They guessed correctly less than half of the time. The subjects all reported that they based their guesses on patterns they believed they saw in the sequence. Instead, if they had simply guessed that the next image would always appear at the top, they would have been correct most of the time.

Summary
Images shown to subjects in an experiment usually showed up at the top of the screen but sometimes showed up at the bottom.
When asked to guess where the next image would appear, the subjects were correct less than half of the time.
Their guesses were based on patterns they thought they saw.
If they always guessed that the image would show up at the top, they would guess correctly most of the time.

Notable Valid Inferences
The images showed up at the top of the screen over 50% of the time.

A
If the subjects had always guessed that the next image would appear at the top, they would not have been basing their guesses on any pattern they believed they saw in the sequence.
Could be false. It could be that the strategy of always guessing that the images would be at the top came from repeatedly observing the image at the top. In the world where the subjects were always guessing top, we don’t have the information to say what this guess was based on.
B
Basing one’s guesses about what will happen next on the basis of patterns one believes one sees is less likely to lead to correct guesses than always guessing that what has happened before will happen next.
Could be false. (B) compares the strategy that the subjects used (basing guesses on perceived patterns) with a strategy that is not mentioned in the stimulus, so we cannot say which of these strategies is more likely to lead to correct guesses.
C
There was no predictable pattern that one could reasonably believe occurred in the series of images on the computer screen.
Could be false. It could be the case that there was a predictable pattern, but that the subjects didn’t pick up on it quickly enough to guess correctly a majority of the time.
D
Some of the subjects sometimes guessed that the next image would appear at the bottom of the computer screen, but were incorrect.
Must be true. Since subjects guessed correctly less than half of the time but would have been right most of the time by always choosing "top," they must have sometimes guessed "bottom" and been wrong. Otherwise, their accuracy wouldn’t be below 50%.
E
The most rational strategy for guessing correctly where the next image would appear would have been simply to always guess that the image would appear at the top.
Could be false. While we know that always guessing top would be more accurate than the strategy that the subjects used, we don’t know that always guessing top would be the most accurate strategy. There could have been other, more accurate, strategies.

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