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time-weighted preservation index

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n. (TWPI, abbr.) ~ A measure of organic material's life expectancy (preservation index) based on the cumulative effects of changes in environment.

Notes: 

The concept of time-weighted preservation index was developed by the Image Permanence Institute.

Citations:
(IPI, PI and TWPI) Nearly every storage environment is dynamic, changing with the weather, with the seasons, or by conscious actions taken to save money or to be more comfortable. It is difficult enough to know the effect of any given static condition on the decay rate of a collection; the total effect of changing conditions over time was impossible to obtain at all, until the conception of the Time-Weighted Preservation Index (TWPI).