Nancy Linsangan found CK-12 Foundation:

• Significantly improves attainment“My students have been able to acquire or attain information and understanding, of class topics, from the CK12 online flexbooks.”
• Significantly builds student knowledge“Higher test scores”
• Significantly improves teaching efficiency and productivity“I am able to assign work right from the CK12 application.”
• Slightly reduces teacher workload“I can assign work from the CK12 application, but not from my school's LMS, Schoology. I have been waiting for a few weeks for a "secret key" (?).”