Catch up on Part 1 of “Space Utilization: Show Your Work!” here.

So, how did you come up with your “adjusted formula” for space utilization?

I had to rewrite the formula so that an average of 30 students per classroom was the “true” maximum limit before CPS considered an elementary school to be overcrowded in 2011-2012.

I double-checked with other large school districts to figure out how they deal with this issue and quickly found NYC School District’s 2010-2011 Utilization and Capacity Report which spells out how they adjusted for classroom maximum limits on Page 5.

For example, in 4th-5th grades, NYC used their maximum classroom limit (28 students) as their TARGET capacity, but calculated backwards to determine a programming efficiency capacity of 75-90% of target/maximum.

CPS’ 2011-2012 formula did not start at the maximum and calculate backwards to the midpoint.  It started at the maximum and loaded on 20% more.


I was curious to see what the formula would have looked like if I began at the maximum and calculated backwards to the midpoint (and to the low point where under-utilization is determined.)  This is what I came up with (with some double-checking assistance from the fabulous Josh Kalov of Open City):

Top of Range = Average of 30 students per classroom.

Take the current formula and place 30 as the “top” of the range.

A x 120% = 30

Solve for A.

30/120% = 25

25 is the correct Ideal IF 30 is the actual Top of the Enrollment Range before a school is considered to be overcrowded.

Therefore, the corrected formula would be:

(Total # of Classrooms x 76.9) x 25 students per homeroom = Ideal Enrollment

Ideal Enrollment x 120% = Top of the Range

Ideal Enrollment x 80% = Bottom of the Range

These  “Top” and “Bottom” of the Range formulas can be double-checked by plugging in the range numbers from any of CPS’ Space Utilization reports from Individual Schools.  For example, if we take Edison RGC’s Space Utilization numbers from 2011-2012:

Does 360 x 120% = 432?  Yes.

Does 360 x 80% = 288?  Yes.


So how does this change the number of schools that CPS labels as under-utilized and overcrowded?

The 2011-2012 Space Utilization formula skews the maximum number of kids in classrooms UP by 20%.

This means that there has to be an average of 20% more students in an elementary homeroom before a classroom is considered overcrowded.

This also means elementary homerooms are determined as under-utilized when they contain less than 24 students. 

If CPS really adhered to their average amount of students (30) in elementary homeroom as their maximum:

Anything over an average of 30 students per elementary classroom would be overcrowded.

Anything under an average of 20 students per elementary classroom would be underutilized.



25 x 120% = 30

25 x 80% = A

Solve for A.

A = 20

20 is the actual low end of the efficiency range if the average classroom maximum is 30 students.

Translated into K-3 and 4-8 grades?  The lowest enrollment an elementary school can have BEFORE it is classified as under-utilized is:

An average of 18 students for grades K-3

An average of 21 students for grades 4-8


Here is what the CPS Space Utilization Formula looked like in 2011-2012 with Classroom Size limit as Ideal Enrollment.


Here is what the Apples to Apples Adjusted Formula looks like for 2011-2012 with Classroom Size limit as maximum enrollment:


Question: Is this all a big conspiracy on the part of CPS to raise classroom maximum limits?

Personally, I try to never attribute to villainy what can be explained by error until I have evidence of intent. Let’s wait to see what the new formula for 2012-2013 is, and make sure that they share how they calculated it and the data that fed into those calculations.

Question: Are you trying to “get” at the Central Office by releasing this?  What if you’re wrong?

Let’s be clear.

I didn’t WANT to be correct about this.  I don’t WANT to believe that there were some schools within CPS that have really reasonable class sizes or school sizes (related to research on class size and physical space for learning) that, if the numbers are the same in 2012-13 as they were in 2011-12, are in danger of being closed.

I don’t want to believe that CPS would let whole wings of school buildings lie vacant when they could have been using that space to co-locate special programs or community organizations that provide wrap-around services.

I don’t want to believe that 77 seats at Lenart Classical SEES Elementary went unfilled last year when there are so many parents seeking educational options for their kids.

And I don’t want to believe that some schools as so overcrowded that they are meeting in hallways to be able to secure a space for learning.

I don’t want to believe that. But I am trying to get a more accurate picture of what is happening at our CPS schools so we have better information for decision-making.

I would be happy to be wrong.  So very happy. Show me the specific numbers for each school and the formula so parents can check those numbers against their own school. I welcome it.

UPDATE: December 4, 2012 about 4:30 pm– Shoot. I wasn’t wrong.  CPS is really using an average of 36 students per classroom as the “new” maximum limit for CPS schools.  Your school won’t be considered overcrowded by CPS unless every homeroom but one has an average 36 kids, and the last room has 37 kids crammed into it. 

And they will consider your school to be underutilized even if there is an average of 24 kids in every homeroom. Using this measure, so many schools in the State of Illinois would be consider “underutilized” and at risk of closure. An average of 24 kids per homeroom is well over the average State of Illinois homeroom (that average in 2011-2012 was 21 kids per elementary homeroom.)

Dear CPS, Chicago’s parents want to give you benefit of the doubt.  But this kind of shell game with numbers is exactly what erodes trust between public school stakeholders.  Would you like to go back to that formula and try again now? Or are you really intent on driving every family who can afford it out of Chicago and CPS altogether?

Get the link to our Raise Your Hand Apples to Apples Space Utilization Dataset here.

Explore the visualization of some of the data here.

One thought on “Space Utilization “Show Your Work!” (Part 2)

  1. Pingback: Space Utilization “Show your work!” (Part 1) | Apples 2 Apples in Chicago Public Schools

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