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Predictive-policing systems are imperfect, better at finding patterns of burglary than of, say, murder.

I know that "better at finding patterns of burglary than of, say, murder." is describing predictive-policing systems, but is it correct to write it as

Predictive-policing systems are imperfect and are better at finding patterns of burglary than of, say, murder.

Can someone explain the meaning difference between these two versions of sentences?

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  • It looks like a confused sentence to me. The second part, no matter how it's written, doesn't logically follow from the first part. In order to make any sense of it, one way of rephrasing it could be: Predictive-policing systems are imperfect, and they analyze different subjects with different levels of accuracy. For example, they are better at finding patterns of burglary than of, say, murder. (A semicolon could be used to keep it as a single sentence, but it seems simpler as two.)
    – Jason Bassford
    Commented Jun 7, 2018 at 8:37
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    Here, 'better at finding patterns of burglary than of, say, murder' is a parenthetical, an adjectival (describing the subject [referent] 'Predictive-policing systems') in apposition to the adjective 'imperfect' and detailing at least one area of imperfection. As it is a parenthetical, a single comma or dash may be used to offset (as the parenthetical is terminal), or a pair of brackets. I prefer the more lightweight comma. Zero punctuation is not an option in this case.
    – Edwin Ashworth
    Commented Jun 7, 2018 at 10:30

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Your first usage, with the comma, may be better written with a colon. Predictive-policing systems are imperfect: better at finding patterns of burglary than of, say, murder. The meaning is then more clear. The subordinate clause explains how the systems are imperfect.

With the "and are," the meaning changes subtly. You create two independent clauses with parallel structure, joined by the conjunction "and." You could expand it like this to understand the difference in meaning. Predictive-policing systems are imperfect. Predictive-policing systems are better at finding patterns of burglary than of, say, murder. In this construction, the second part is not necessarily explanatory of the first part. The imperfection might be unstated: buggy software or high cost. Using a colon clarifies the semantic intent: that you are explaining the imperfection.

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    I'm a fan of the em dash in cases like this—you can't go wrong! Chicago states: "The em dash...is the most commonly used and most versatile of the dashes. [They're] used to set off an amplifying or explanatory element and in that sense can function as an alternative to parentheses, commas, or a colon..."
    – Jas. MacOisdealbha
    Commented Jun 7, 2018 at 14:11
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This is a common and typical pattern of structuring such sentences. It has a complex meaning.

Predictive-policing systems are imperfect, better at than of, say, murder.

  1. Predictive-policing systems are imperfect.
  2. Being imperfect, they are only suited to certain types of applications and not to others.
  3. Finding patterns of burglary is one type of application. It does not need a perfect system.
  4. (Solving a case of) murder is another type. It is not amenable to an imperfect system like predictive-policing.

HTH.

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