"Margin of Error" is the sixth episode of the fourth season of the HBO original series The Wire. Written by Eric Overmyer from a story by Ed Burns & Eric Overmyer, and directed by Dan Attias, it originally aired on October 15, 2006.

On Sunday morning, Mayor Royce and Councilmen Gray and Carcetti attend church to win support from the ministers as the election approaches. Later, while Carcetti and Wilson are campaigning downtown, State Delegate Watkins shows a flier linking Carcetti to a notorious slumlord. Wilson recognizes the flier as a last-minute smear tactic by Royce, but Carcetti becomes agitated that the attack might work. Carcetti, Wilson, D'Agostino, and Gerry discuss how to respond to the flier. At Wilson's urging, Carcetti appears on a radio talk show and feels more positive from when it proves successful. Senator Davis offers to support Carcetti and split Royce's vote in exchange for $20,000. Carcetti reluctantly agrees.

Sydnor and Herc watch as Marlo meets with his lieutenants. Marlo makes a phone call about pick up a "skinny girl from New York," which Herc interprets as an arrangement to pick up a supply of cocaine. Herc follows Marlo to Penn Station and has arrested when he meets a woman who turns out to not be carrying drugs. Marlo has engineered the incident to draw out the police. Meanwhile, Greggs learns that a prisoner has offered information on the dead state's witness. However, Rawls tells Landsman to detail Greggs and Norris to uniformed duty at the polls, stalling the witness case until after the election. At school, Prez offers to help Dukie get clean clothes. Donnelly and Grace remove Namond from regular class and puts him in Colvin and Parenti's study program along with nine other students. When Randy is brought to Donnelly concerning an altercation with a female student, he reveals that he knows about a murder. Later, Prez appeals to Donnelly to let him hand Randy's confession on to someone that he trusts in the police.

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