Jacob Kramer found CK-12 Foundation:

• Moderately improves attainment“Students have additional practice with the material to strengthen their understanding of the material.”
• Significantly builds student knowledge“Students can read lessons on concepts that they may struggle with at their own pace to gain a deeper understanding of the material. The adaptive practice is also a great way for students to test their knowledge and let them know if they need additional practice with the material.”
• Significantly improves teaching efficiency and productivity“My school is currently on a hybrid model and CK-12 has been a great resource for the students on their days away from school. The material is still challenging for the students and a great way to strengthen their understanding of material we discussed in class or introduce them to concepts that we are going to discuss the next day they are in school.”
• Significantly reduces teacher workload“The class reports feature is a great way to assess students' performance on a variety of assignments at a mere glance. The website is user-friendly for both myself and my students.”
• Significantly saves school money“In the near future, I can see myself using CK-12 as a free virtual textbook for my Biology classes.”