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The Star Trek: Deep Space Nine book Legends of the Ferengi says Starfleet security personnel "rarely survive beyond the second act break".
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Most casualties were security personnel, whose uniform was red. Of the 55 crew members killed in the series, 24 were wearing red shirts, compared to 15 who had unconfirmed shirt colors, 9 in gold shirts, and 7 in blue shirts. The first instance of what now is an established trope can be seen in the episode " What Are Little Girls Made Of?" (1966). In Star Trek, red-uniformed security officers and engineers who accompany the main characters on landing parties often suffer quick deaths.
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