Existing computationally tractable duration models impose onerous restrictions on the hazard function’s shape. A theoretically consistent, tractable and yet flexible hazard function capable of exhibiting constant, monotonically increasing or decreasing. U-shaped, and inverted U-shaped forms is proposed.

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“Expo-Power: A Flexible Hazard Function for Duration Data Models,” A. Saha and L. Hilton, Economics Letters, 54 (1997), 227–233