Spaced recognition is essentially just flashcard software, but flashcard software backed with an intelligent algorithm that attempts to find the optimally efficient timing to aid your memory.
This timing all arises from the idea of "memory schedules" put forth by people such as Paul Pimsleur, who gave his name to a popular series of cassette-based language courses in the 1960s (a series which continues to be published to this day with virtually no differences to the basic structure).
The first principle behind these memory schedules is that new information is gradually forgotten, and needs to be reminded. (So far, so obvious.)
The second principle is that the better learned something is, the longer it takes before being fully forgotten -- the memory pattern is stronger. (Also obvious.)
Taken together, these principles give the basic form of memory schedules: revise the item to be remembered just late enough so that it's not forgotten, at progressively longer intervals.
Now this of course is obvious, and all teachers schedule their revision along similar principles. The real promise of memory schedules is the ability to get a measurable and verified number on it.
So the hard liners within memory schedules did put figures in place, completely ignoring the fact that some things are more difficult to remember than others -- for example, memorising the full 10 or 11 digit phone number for someone in a foreign country is going to be harder than memorising the number of your neighbour, whose number will probably only differ from yours by 3 or 4 digits.
The numbers, therefore can't be right, which is why more recent memory researchers have been looking for formulas that can take into account complexity. And as it's hard to objectively measure that complexity beforehand, very sensibly, SRS tries to work it out for each item to be learned as they go, based on the learner's performance. That's a good compromise.
But there's another claim from the hardliners that's really very hard to stomach: the claim that nearly forgetting and then reminding makes for a stronger memory trace. Than what? Than revising it more frequently.
Less revision = better learning...?That is a weird claim, and the guys that claim it must presumably have data to back it up, but I just can't see how it can be a generally applicable truth.
What is definitely true is that excessively frequent repetition can keep something in short-term memory without ever forcing it to be stored in, or recalled from, long-term memory; and something that isn't in long-term memory isn't learned. Worse: if you are really frequent in your repetition, you can hold the information in working memory, and never have to recall it at all.
This is, of course, a problem that should be familiar to most language teachers and learners. The first hour of language instruction is often very light. In a one-hour introduction to Finnish, I was taught to say "good morning", "how are you?", "I'm fine,", "what is your name?", "My name is...", "what is it?" (or maybe "what is this?" or "that?"), "it's a..." and 6 or 8 proper nouns (including "car", "key" and "aeroplane"). I forgot everything except "good morning" and "key" within about 24 hours. I now only remember "good morning". Everything was repeated too quickly, so nothing went into long-term memory.
But that is not to say that repeating at the very last minute is the correct answer, and this flies in the face of a lot of material on memory anyway, and particularly in terms of language.
There is a simple rule in memory: the more often you are called on to recall something, the quicker and easier it becomes to recall. To use a trivial example, "is" is far quicker to recall than "carbunkle", because we use "is" every single day of our lives. (Or the equivalent in our language, if not English speakers.)
If not reaching that threshold of "nearly forgetting" inhibited better memorisation, then we would be in the paradoxical situation of knowing words we almost never say better than the words we say every day, but I have never said "what's the word... it's on the tip of my tongue... oh yes... is." It just doesn't work that way.
So no, SRS isn't the optimal way to remember individual items, but it's certainly a pretty efficient way to learn a bulk load of items.
It's good, but it doesn't deserve the hard line.