The study period approach to comparing unequal-lifespan alternatives picks a fixed planning horizon (the study period) and evaluates every alternative over that same horizon. If an alternative’s natural life is longer than the study period, it gets truncated and any remaining value at horizon-end is captured as a salvage value. If shorter, it gets repeated (or replaced) within the period.
For example: comparing a 5-year machine and a 12-year machine over a 10-year study period. The 12-year machine runs for 10 years and salvages at year 10 with whatever market value it would have then. The 5-year machine runs full life (years 1-5), gets replaced with an identical unit (years 6-10), with that second unit also salvaged at year 10.
The advantage is honesty about commitment: most firms don’t actually plan to operate an asset for its full engineering life — they plan against a specific business horizon (a contract term, a product lifecycle, a strategic review point). The study period reflects that.
The challenge is estimating salvage values mid-life. Market value of an asset depends on age, condition, technology generation, and demand. Forecasting it accurately is harder than forecasting full-life salvage. The cleaner the resale market for the asset (cars, generic office furniture, common industrial equipment), the more reliable the salvage estimate.
When the alternatives’ costs and benefits are well-distributed over time (no big bumps near the end of life that the study period would chop off), study-period truncation gives an honest comparison. When there are end-of-life cliffs (big overhaul costs near year 12 for the 12-year machine), truncating at year 10 hides them — and you may underestimate the long-life option’s true cost.
Alternative methods for unequal lifespans: Repeated lives approach (repeat to LCM) and Annual worth method (convert to per-year and compare). Each makes its own implicit assumptions; the right choice depends on which assumptions match your planning context.
For the broader topic see Comparison of alternatives.