Tamara Guion-Yagy was disappointed when Tetra Tech, an environmental engineering firm in Pasadena, Calif., hired somebody else for the job that she wanted. The 40-year-old graphic designer thought she was being tried out when the firm hired her as a temporary worker.

So Ms. Guion-Yagy worked even harder at the same temp job, often staying late to finish work. Her manager responded by creating another full-time position for Ms. Guion-Yagy. “I knew I’d be good at the job and liked the work,” she says. “I just needed to show them how much.”

When times are prosperous, companies are more likely to use temporary jobs as a low-risk way to vet full-time candidates. But the conversion rate from temporary to permanent worker has been low over the past few years as more companies lean on temps as a hedge against a double-dip recession, says Jonas Prising, president of Manpower North, a temporary-staffing company in Milwaukee, Wis.

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