"Existing Collection Strength" and shelflist count correlations in RLG's conspectus for music

"Existing Collection Strength" (ECS) is an estimate of the extent to which subject areas in Research Libraries Group (RLG) collections meet research level objectives on a scale of 0 to 5. ECS can be interpreted as a proxy for shelflist counts. To test that assumption, two main sets of correlations were computed: 1) 17 correlations for 17 RLG libraries across 138 LC class ranges, indicating how well the scale represents collection strength within a library; 2) 138 correlations for 138 LC ranges across 17 libraries, indicating how well those ECS estimates were made from library-to-library. "Within library" correlations ranged from 0.36 to 0.89, all significant, with a confidence interval of the mean (0.66) from 0.55 to 0.74. "Across library" correlations ranged from -0.34 to 0.94, with none significant below 0.47. The mean was 0.54, with a confidence interval 0.08 to 0.81. As a proxy variable for shelflist within libraries, ECS is expeditious but, necessarily, less than perfect, and is, therefore, a trade-off between convenience and accuracy. However, the wide range of correlations across libraries, from inverse to highly positive, indicates that using ECS to compare libraries is not appropriate, probably because each library interprets and applies the ECS scale differently.