I want to explain why LJ Index scores are not well-behaved. That is, why they don’t conform to neat and tidy intervals the way HAPLR scores range from about 30 to 930. HAPLR scores fall into a predictable range because they are built on percentiles. Any given library’s score is a sum of 15 percentile rankings, one for each statistical item HAPLR uses (like circulation per visit). As you probably know, percentiles range from from 99th down to zero(th).  (Nobody can be in the exact 100th percentile for reasons I’ll skip here.)  If a library ranks at the 99th percentile for all 15 HAPLR items, that library earns a score of 990. If it ranks at the lowest (0th) percentile for all of the items, then it gets a score of zero. In reality, libraries don’t get all high or all low rankings on the 15 HAPLR items—they get a mixture.   [Read more…]