PrepTest 119, Section 2, Question 22
Detective: Laser-printer drums are easily damaged, and any(49 more words not shown)
Which one of the following,(7 more words not shown)
- ACriminals are unlikely(10 more words not shown)
- BDrum nicks are(14 more words not shown)
- CThe manufacturing process(8 more words not shown)
- DBlemishes on documents(11 more words not shown)
- EMost suspicious documents(6 more words not shown)
Reading the argument
What if my printer and my neighbor's printer leave the same mark?
Let's accept that a nick in a printer's drum leaves a matching blemish on each page. That explains how damage to a printer can show up on its output. It doesn't establish that every printer has a unique pattern of damage.
The detective wants to identify the precise printer, not just find a printer that could have produced the document. If several drums have the same nick, the page could match all of them. A matching mark would narrow the possibilities without identifying one machine.
What we’re looking for
For a weakener, I'd look for that shared damage: different printers can have the same nick and leave the same blemish. The detective is treating these marks like individual fingerprints without establishing that they distinguish one printer from another.
Working the answers
- A
Whose printer the criminal uses is a different question. Suppose a criminal uses a neighbor's printer and the method correctly identifies that machine. It has still done what the detective claims: traced the page to its printer. Whether that also identifies the criminal isn't the conclusion we're testing.
- B
Requiring skill doesn't make a method unreliable. A trained examiner might accurately determine a nick's size and shape, perhaps with magnification. We'd need a reason to think that even a correctly observed match fails to identify the particular printer. B doesn't give us one.
- CCorrect
Several drums often receive the same nick during manufacturing. Those printers can therefore produce matching blemishes even though they are different machines.
Now finding a drum with the right nick doesn't settle where the document came from. It might have come from that printer, or from another printer with the same manufacturing damage. This directly attacks the uniqueness the detective needs.
- D
A hidden blemish can prevent us from making a match in the first place. The detective's inference concerns what we can conclude when we do match a visible blemish to a nick. D identifies a situation in which the method can't be used; it doesn't show that an observed match points to the wrong printer or to several possible printers.
- E
We are discussing suspicious documents that were laser-printed. The detective doesn't claim this method works on every suspicious document. Even if most documents fall outside that category, the method could still reliably identify the printer for the documents it covers.
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