The following charts show how often different types of library books are checked out, on average, per book, for each school level. The averages were caluclated using the last 3 years of circulation history, encompassing more than 3.2 million checkouts.
English Books : 
Easy
Level  1 Year Yotal  Year to Date 
ECE  1.6182 times  1.6182 times 
Elementary  0.9568 times  0.9568 times 
K8  0.9580 times  0.9580 times 
Middle  0.7532 times  0.7532 times 
612  0.2320 times  0.2320 times 
High  0.1236 times  0.1236 times 

Easy Fiction
Level  1 Year Yotal  Year to Date 
ECE  1.5546 times  1.5546 times 
Elementary  0.8851 times  0.8851 times 
K8  1.1222 times  1.1222 times 
Middle  1.1973 times  1.1973 times 
612  0.4479 times  0.4479 times 
High  0.2104 times  0.2104 times 

Easy NonFiction
Level  1 Year Yotal  Year to Date 
ECE  0.6429 times  0.6429 times 
Elementary  0.6887 times  0.6887 times 
K8  0.8826 times  0.8826 times 
Middle  0.2632 times  0.2632 times 
612  0.2424 times  0.2424 times 
High  0.2069 times  0.2069 times 

Biography
Level  1 Year Yotal  Year to Date 
ECE  0.8986 times  0.8986 times 
Elementary  0.3716 times  0.3716 times 
K8  0.2298 times  0.2298 times 
Middle  0.1928 times  0.1928 times 
612  0.0865 times  0.0865 times 
High  0.0735 times  0.0735 times 

Fiction
Level  1 Year Yotal  Year to Date 
ECE  1.7556 times  1.7556 times 
Elementary  1.1552 times  1.1552 times 
K8  0.9379 times  0.9379 times 
Middle  0.5590 times  0.5590 times 
612  0.2719 times  0.2719 times 
High  0.2028 times  0.2028 times 

NonFiction
Level  1 Year Yotal  Year to Date 
ECE  1.2627 times  1.2627 times 
Elementary  0.8382 times  0.8382 times 
K8  0.5883 times  0.5883 times 
Middle  0.2785 times  0.2785 times 
612  0.1237 times  0.1237 times 
High  0.0859 times  0.0859 times 

Reference
Level  1 Year Yotal  Year to Date 
ECE  2.9167 times  2.9167 times 
Elementary  0.3625 times  0.3625 times 
K8  0.0971 times  0.0971 times 
Middle  0.1478 times  0.1478 times 
612  0.0176 times  0.0176 times 
High  0.0437 times  0.0437 times 

Story Collection
Level  1 Year Yotal  Year to Date 
ECE  1.1333 times  1.1333 times 
Elementary  0.9548 times  0.9548 times 
K8  0.4964 times  0.4964 times 
Middle  0.3071 times  0.3071 times 
612  0.1671 times  0.1671 times 
High  0.1061 times  0.1061 times 

Spanish Books : 
SPA Easy
Level  1 Year Yotal  Year to Date 
ECE  0.3596 times  0.3596 times 
Elementary  1.1842 times  1.1842 times 
K8  1.5833 times  1.5833 times 
Middle  0.3272 times  0.3272 times 
612  0.2176 times  0.2176 times 
High  0.0412 times  0.0412 times 

SPA Easy Fiction
Level  1 Year Yotal  Year to Date 
ECE  0.5000 times  0.5000 times 
Elementary  1.1706 times  1.1706 times 
K8  1.9339 times  1.9339 times 
Middle  0.1250 times  0.1250 times 
612  0.2593 times  0.2593 times 
High  0.2564 times  0.2564 times 

SPA Easy NonFiction
Level  1 Year Yotal  Year to Date 
ECE  0.0000 times  0.0000 times 
Elementary  0.6757 times  0.6757 times 
K8  0.8973 times  0.8973 times 
Middle  0.0000 times  0.0000 times 
612  0.4667 times  0.4667 times 
High  0.0000 times  0.0000 times 

SPA Biography
Level  1 Year Yotal  Year to Date 
Elementary  0.2017 times  0.2017 times 
K8  0.4274 times  0.4274 times 
Middle  0.2990 times  0.2990 times 
612  0.1697 times  0.1697 times 
High  0.0658 times  0.0658 times 

SPA Fiction
Level  1 Year Yotal  Year to Date 
ECE  0.6667 times  0.6667 times 
Elementary  0.9678 times  0.9678 times 
K8  0.7964 times  0.7964 times 
Middle  0.4716 times  0.4716 times 
612  0.2757 times  0.2757 times 
High  0.1749 times  0.1749 times 

SPA NonFiction
Level  1 Year Yotal  Year to Date 
ECE  0.1776 times  0.1776 times 
Elementary  0.5707 times  0.5707 times 
K8  0.5753 times  0.5753 times 
Middle  0.2826 times  0.2826 times 
612  0.1592 times  0.1592 times 
High  0.0608 times  0.0608 times 

SPA Reference
Level  1 Year Yotal  Year to Date 
Elementary  0.0675 times  0.0675 times 
K8  0.0222 times  0.0222 times 
Middle  0.0310 times  0.0310 times 
612  0.0330 times  0.0330 times 
High  0.0745 times  0.0745 times 

What are they?
Expected Checkout Rates are estimates of how many books an average librarian would check out at a particular school based on the types and numbers of books in that collection.
Why do we use them?
We have found that schools with more books per student also check out more books per student, on average. Therefore, comparisons of checkout rates between two schools can be biased if one school has a larger collection than the other. Instead we prefer to compare a school's checkout rate to how we would expect an "average" librarian to perform with that particular collection.
How are they calculated?
We took three years of checkout information and determined that average number of times each type of book was checked out at each school level (e.g. Elementary schools, Middle schools, etc.) These average circulation rates, which can be seen in the section above, are then multiplied by the number of books for each type at the school, and divided by the number of students there.
For example, Fiction books at Elementary schools are checked out 1.19 times per year, on average. So if Elementary School "X" has 500 Fiction books and 200 students, we would expect the average librarian at this school to check out 2.975 Fiction books per student. ( (500 X 1.19) / (200) )
We then add of the expected rates for each type of book to determine the overall expected rate for that school, and then apply an aging factor to account for the disparate rates of circulation for newer versus older books. This final "expected checkout rate" is then compared to a school's actual rate to provide a neutral, biasfree basis of evaluation.