Thomas Freeman found CK-12 Foundation:

• Significantly improves attainment“Students can use the material to review topics, use it as a textbook, and get more engaged using the simulations and PLIX.”
• Significantly builds student knowledge“Students get a chance to learn new vocabulary and work towards mastery of a topic.”
• Significantly improves teaching efficiency and productivity“CK-12 makes assigning mastery-based homework easy and makes it easy to access free learning resources anywhere.”
• Significantly reduces teacher workload“The self grading mastery-based assignments make it easy for students to practice frequently and often without increasing the amount of manual grading.”
• Significantly saves school money“All the resources are free online.”