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What: Shares of human resources specialist Insperity (NYSE:NSP) were falling sharply today, down as much as 16%, after its guidance missed in its earnings report.

So what: In the fourth quarter, Insperity beat EPS estimates by $0.02, with a $0.47 per-share profit, but fell short on revenue. Still, shares traded essentially flat until management provided guidance in the conference call, after which they fell 10% in just 20 minutes and continued sliding for the rest of the day. Management said it expected 2013 earnings to be just $1.47 to $1.56, due, in part, to reduced interest income, investment in health-care reform strategy, and a lower January starting point. Analysts had been expecting earnings of $1.66 a share.

Now what: With changes in payroll taxes and other work-related costs like health-care reform, Insperity seems headed for a more challenging 2013 than expected, but the overall business model still looks sound. The stock may still be pricy even after today's fall, but the long-term picture looks good for Insperity, especially as the job market improves.

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Fool contributor Jeremy Bowman has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.