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Understanding TWPI

The Time Weighted Preservation Index

From user community feedback we bring you a newly updated metric. This metric previously known as the “Damage” index has been upgraded to “TWPI” (the Time-Weighted Preservation Index). This tool is an industry standard developed by the Image Permanence Insitute, and our transition to this metric aims to help close communication gaps between the data and those who use it to make decisions.

As always, thank you for your feedback!

Team Conserv

Key Performance Indicators, or KPIs, should help us answer the simple question "How are we doing?" -- read more about our KPI philosophy in our article "Understanding Preservation KPIs".

Take the example of the common temperature and relative humidity chart so common in environmental monitoring reports.

What do these conditions mean for the longevity of the objects in the collection? Are these conditions causing damage or not? How much damage? Hard to tell by just looking at the temperature and relative humidity data alone, but when brought together things become more clear.  This is where the TWPI tool steps in.

TWPI, Time-Weighted Preservation Index

The Time-Weighted Preservation Index KPI Is based on research conducted by the Image Permanence Institute. TWPI uses damage rate calculations to express the longevity of an object (in years) in its current environment.

TWPI calculations are typically done with at least one year of data, so we recommend using TWPI only with datasets of 1 year or longer.

Understanding TWPI

 TWPI Value
1 - 45
Poor Environment
45 - 75
OK Environment
75 - 100
Good Environment
Excellent Environment

Table from:  Nishimura, D. W. (2011, April). Understanding Preservation Metrics. Rochester; Image Permanence Institute.

The value displayed is a snapshot of the data that has been aggregated and calculated for you over the selected period of time.  This is a simplified value that can be understood by all across institutions and conservation knowledge levels.  The value is given in years to allow for a clear understanding as to the life expectancy of a given object in the surrounding environment.

If you are interested in looking at the formula and calculation for TWPI, you can find it in this 2023 Open Access Article: Context, Development, and Intent: An Introduction to the IPI Preservation Metrics by by Emma J. Richardson, Marvin Cummings, and Jean-Louis Bigourdan.

IPI Example

"If a storage condition has a PI of 100 years, it means that a problem object like acidic paper would require 100 years to discolor to the same extent that it would in 50 years at room temperature and moderate RH. PI values can still be used in a purely relative sense (Nishimura, 2011)."

The approach to the Time-Weighted Preservation Index (TWPI) is best laid out in IPI's  document: "Understanding Preservation Metrics"

An interesting critique of the TWPI is laid out in Tim Padfield's article, "The Preservation Index and the Time Weighted Preservation Index"

If you are interested in a deeper dive on the history and use of TWPI in the preservation community, please see our blog post

As always if you have questions or have insight that might be helpful to your fellow users, please join our Conserv Community

Have a more pressing question or problem? Check out our Knowledge Base articles or Create a Support Ticket.