Great summary on how to measure retention using Amplitude.
https://s3-us-west-2.amazonaws.com/secure.notion-static.com/af7198e9-0f5c-4535-8788-41120f71bacc/Amplitude_Analytics-playbook-vol1_20181213.pdf
https://s3-us-west-2.amazonaws.com/secure.notion-static.com/dc218853-522b-42ca-8485-f8f24df0800e/Mastering_Engagement_Playbook_02_26_19_Amplitude.pdf