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  1.3435 times  1.3435 times 
Elementary  1.1031 times  1.1031 times 
K8  1.2174 times  1.2174 times 
Middle  1.1316 times  1.1316 times 
612  0.3403 times  0.3403 times 
High  3.5032 times  3.5032 times 

Easy Fiction
Level  1 Year Yotal  Year to Date 
ECE  0.9542 times  0.9542 times 
Elementary  0.9672 times  0.9672 times 
K8  1.4771 times  1.4771 times 
Middle  1.4467 times  1.4467 times 
612  0.3529 times  0.3529 times 
High  1.4248 times  1.4248 times 

Easy NonFiction
Level  1 Year Yotal  Year to Date 
ECE  0.2619 times  0.2619 times 
Elementary  0.8714 times  0.8714 times 
K8  1.0750 times  1.0750 times 
Middle  0.6316 times  0.6316 times 
612  0.1389 times  0.1389 times 
High  0.2730 times  0.2730 times 

Biography
Level  1 Year Yotal  Year to Date 
ECE  0.5238 times  0.5238 times 
Elementary  0.4047 times  0.4047 times 
K8  0.2817 times  0.2817 times 
Middle  0.2902 times  0.2902 times 
612  0.1196 times  0.1196 times 
High  0.0893 times  0.0893 times 

Fiction
Level  1 Year Yotal  Year to Date 
ECE  1.2153 times  1.2153 times 
Elementary  1.1807 times  1.1807 times 
K8  1.0271 times  1.0271 times 
Middle  0.8354 times  0.8354 times 
612  0.3328 times  0.3328 times 
High  0.3278 times  0.3278 times 

NonFiction
Level  1 Year Yotal  Year to Date 
ECE  0.9804 times  0.9804 times 
Elementary  0.9294 times  0.9294 times 
K8  0.7106 times  0.7106 times 
Middle  0.4054 times  0.4054 times 
612  0.1555 times  0.1555 times 
High  0.1468 times  0.1468 times 

Reference
Level  1 Year Yotal  Year to Date 
ECE  2.3611 times  2.3611 times 
Elementary  0.4078 times  0.4078 times 
K8  0.1090 times  0.1090 times 
Middle  0.1805 times  0.1805 times 
612  0.0304 times  0.0304 times 
High  0.0615 times  0.0615 times 

Story Collection
Level  1 Year Yotal  Year to Date 
ECE  0.9333 times  0.9333 times 
Elementary  1.1675 times  1.1675 times 
K8  0.7135 times  0.7135 times 
Middle  0.4333 times  0.4333 times 
612  0.2065 times  0.2065 times 
High  0.1313 times  0.1313 times 

Spanish Books : 
SPA Easy
Level  1 Year Yotal  Year to Date 
ECE  0.3694 times  0.3694 times 
Elementary  1.2849 times  1.2849 times 
K8  1.9938 times  1.9938 times 
Middle  0.4568 times  0.4568 times 
612  0.3035 times  0.3035 times 
High  5.0383 times  5.0383 times 

SPA Easy Fiction
Level  1 Year Yotal  Year to Date 
ECE  0.3333 times  0.3333 times 
Elementary  1.0408 times  1.0408 times 
K8  2.3794 times  2.3794 times 
Middle  0.0417 times  0.0417 times 
612  0.3333 times  0.3333 times 
High  6.3846 times  6.3846 times 

SPA Easy NonFiction
Level  1 Year Yotal  Year to Date 
ECE  0.0000 times  0.0000 times 
Elementary  0.7421 times  0.7421 times 
K8  1.1529 times  1.1529 times 
Middle  0.0000 times  0.0000 times 
612  0.5333 times  0.5333 times 
High  1.3333 times  1.3333 times 

SPA Biography
Level  1 Year Yotal  Year to Date 
Elementary  0.2415 times  0.2415 times 
K8  0.5119 times  0.5119 times 
Middle  0.4365 times  0.4365 times 
612  0.2468 times  0.2468 times 
High  0.1116 times  0.1116 times 

SPA Fiction
Level  1 Year Yotal  Year to Date 
ECE  0.5556 times  0.5556 times 
Elementary  0.8287 times  0.8287 times 
K8  0.7604 times  0.7604 times 
Middle  0.9148 times  0.9148 times 
612  0.3093 times  0.3093 times 
High  0.3555 times  0.3555 times 

SPA NonFiction
Level  1 Year Yotal  Year to Date 
ECE  0.1402 times  0.1402 times 
Elementary  0.6133 times  0.6133 times 
K8  0.6737 times  0.6737 times 
Middle  0.6946 times  0.6946 times 
612  0.1790 times  0.1790 times 
High  0.5981 times  0.5981 times 

SPA Reference
Level  1 Year Yotal  Year to Date 
Elementary  0.1036 times  0.1036 times 
K8  0.0268 times  0.0268 times 
Middle  0.0349 times  0.0349 times 
612  0.0196 times  0.0196 times 
High  0.0714 times  0.0714 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.