Spaced Repetition is the most effective method known for Stage 2 (Absorption) learning.
Here's why that's the case, and who says so. (Besides us.)
Learning is a multi-stage process. Stage 2 - Absorption - is the most difficult, and most time consuming part of learning. It is the stage that nobody else helps you with - neither your teachers, your peers or anyone else. It is the stage that students are least equipped to manage on their own.
Spaced Repetition (SRS) is the most effective learning method for the Absorption stage. SRS can massively reduce the effort required for, and decrease the time spent in, the Absorption stage.
SRS combines the two most effective learning techniques known:
The Practice Testing technique (aka Recall Testing, Retrieval Testing, Active Recall Testing...) tests your ability to remember what you are learning - simply by answering questions about the subject. Practice Testing is commonly implemented via flashcards. Flashcards similar to those with which you practiced the times tables or vocabulary words when you were a child.
Flashcards have been around for a long time. And they come from humble, low tech roots. But make no mistake - they are an extremely effective study technique, with decades of research to back them up.
Distributed Practice is what software adds to the humble flashcard.
For each question that you review, you log how easily you were able to recall the answer. Based on that rating, the SRS system determines how soon you should be retested on that question. It could be a minute later, a month later, or anywhere in between.
The more difficulty you had recalling the answer, the sooner you'll see the question again. Each time you try, you'll reinforce and strengthen the memory.
Together, these two techniques (Practice Testing and Distributed Practice) serve to engage a process known as Overlearning, which "ensures that information is more impervious to being lost or forgotten".
Overlearning dramatically changes the Forgetting Curve, which describes "how information is lost over time when there is no attempt to retain it".
These are simple concepts, yet they are astonishingly effective in practice.
You don't have to take our word for it.
In 2013, Dunlosky et al published a survey of existing research on ten of the most commonly used study techniques. This study reviewed over 700 prior papers. Only two of the ten techniques received the highest ranking:
"We rated two strategies - practice testing and distributed practice - as the most effective of those we reviewed because they can help students regardless of age, they can enhance learning and comprehension of a large range of materials, and, most important, they can boost student achievement."
Article - American Educator magazine, Fall 2013
Paper - Psychological Science in the Public Interest 2013
Spacing Effect - Wikipedia article
Testing Effect - Wikipedia article
Spaced Repetition - Wikipedia article
Spaced Repetition - Gwern Branwen
Testing and Spacing Both Aid Memory - Association for Psychological Science, January 4, 2016
As you can see, the research on the principles extends back decades. The research is extensive, it is deep and it is unequivocal.
SRS itself has been widely available for over 20 years. Yet most students - even most educators - have never even heard of SRS.
It turns out that there's a pretty straightforward reason for why you don't see everybody using it to learn everything - making a good deck is really hard.