Preservation Metrics
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      Time-Weighted Preservation Index

      The Time Weighted Preservation Index

      The Time-Weighted Preservation Index (TWPI) is an industry standard that was developed by the Image Permanence Institute. Metrics provide insight into how you are doing, and this metric is designed to improve clarity and bridge communication gaps between the data and decision makers. For more information about Conserv’s approach to metrics, see Preservation Metrics.
      The following example illustrates the commonly used temperature and relative humidity (RH) charts that are found in environmental monitoring reports.

      Understanding what these conditions mean for the longevity of the collection and whether they are causing damage can be difficult to determine from temperature and RH data alone. However, when these factors are analyzed together, the picture becomes clearer. This is where the TWPI tool proves useful.

      TWPI Calculations

      The TWPI uses damage rate calculations to estimate an object's longevity (in years) within its current environment.

      Tip

      For accurate TWPI results, Conserv recommends that you use datasets that cover one year or more.

      Using the following example, the displayed value represents an aggregated snapshot of the data that was calculated for the selected period. This simplified value is designed to be easily understood across various institutions and levels of conservation knowledge. Expressed in years, it provides a clear estimate of an object's life expectancy in its current environment.

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

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

      "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)

      Additional Resources

      For a TWPI formula and calculation example, see Context, Development, and Intent: An Introduction to the IPI Preservation Metrics by Emma J. Richardson, Marvin Cummings, and Jean-Louis Bigourdan.

      The TWPI is thoroughly explained in the Image Permanence Institute’s document, Understanding Preservation Metrics.

      Tim Padfield’s article, The Preservation Index and the Time-Weighted Preservation Index, offers a compelling critique of the TWPI.

      For a more in-depth exploration of the history of the TWPI and its application in the preservation field, see this blog post.

      If you have questions or insights that could benefit other users, join and post in the Conserv Community.