Spaced Repetition, Scheduling, and Retention
Skritter is a built on a custom Spaced Repetition System (SRS) tailored for Chinese and Japanese.
The system automatically keeps track of duplicate words and makes sure that you don't see them more often, even if you add them from several lists. In a nutshell: you don't need to worry about duplicates-- it's under control!
Spaced Repetition (Explanation)
As an adult, there are two possible strategies for learning Chinese characters: spaced repetition, and anything else. Seriously, the difference is dramatic: research shows that after enough characters, you'll forget more than you learn with naïve study methods. The principle behind spaced repetition is that it's a lot more effective to review something at the point when it's about to be forgotten. The longer you have known it, the less frequently you'll have to review it.
Ideally, you want to review things when you have a 90% chance of remembering them (corresponding to a 90% retention rate). After several repetitions of a character, your repetitions will be months apart (and soon years apart).
The problem with most spaced repetition systems is that they have no idea whether you remembered an answer unless you tell them. Many of them ask you to grade yourself on a 0-5 scale after each prompt, so that they can adjust your interval accordingly. Not only is that distracting, but it doesn't have active recall built in. Active recall is the key to long-term memory: you have to come up with the answer yourself, rather than just see the answer on the flip-side of the card. Existing programs are spaced repetition for flashcards, not characters. That's why there's Skritter!
Skritter Grading Buttons and Scheduling
A grade of 1 ("don't know") gives you a next review interval about 25% as long as the one that was scheduled for this review, with a minimum of thirty seconds and a maximum of one week. If it's the first time you've seen an item, the default interval is ten minutes.
A grade of 2 ("so-so") gives you a next review interval about 90% as long as the one that was scheduled for this review. If you mark something so-so on the first time you see it, the default interval is 1.4 days.
A grade of 3 ("got it") gives an interval about 2.2 times as long as the one you just had, not the one you were scheduled for, if you studied it on time. If you remembered it while overdue, Skritter guesses that you knew it better than we'd thought, so the next interval is longer. For example, if an item was scheduled for one week, and you saw it after two weeks, you'll get it again in about 3.4 weeks. Conversely, you don't get much of an interval boost for seeing something again before you need to. If an item was scheduled for one week, and you saw it after one day, you'll see it again in about 8.2 days. This means that studying too often doesn't do much, whereas remembering something after a long time is a very efficient memory boost ( if you remember it!). The initial interval for a grade of 3 is one week.
A grade of 4 ("too easy") is like a 3, but with a factor of 3.5 instead of 2.2. The default initial interval is four weeks. You only need to press 4 three times in the first three years for an item, so it's a very quick way to get easy stuff out of the way.
In addition to the above, each interval is randomized by up to +/- 7.5%. Items you get wrong a lot will start to grow even more slowly. Items which you always get right will grow even more quickly. And, most importantly, for each type of prompt (writing, reading, definition, tone), for each general interval range (first time, under twenty minutes, under five hours, under eight days, over eight days), Skritter will adjust these factors based on your past performance in order to target your desired retention rate. So if you often forget tones which have intervals scheduled between five hours and eight days, then Skritter will gradually reduce those intervals. If you don't ever forget how to write things that you knew the first time you saw them, then those initial intervals will get longer and longer.
Each item's scheduling can be affected when you review other parts for its word. If you study the writing for 先生, then if the reading and definition are also due, they will be pushed back into the future somewhat, so that the reviews don't interfere with each other. After all, if you just saw the details for 先生 two minutes ago, it would be too easy to remember them if you got its definition prompt now. So you might see the definition prompt a little later, or the next day, depending on how the length of the current interval. A consequence of this can be that you don't even see the reading or definition parts of a word for days after adding it to My Words, until you've started to remember the writing part. We think this is a good thing.
If you took a quiz on everything you've added to Skritter, your retention rate would be your score, with 95% being a good target. To learn faster, lower your retention rate a bit-- you'll have much fewer reviews due, but you won't know quite as much of the material at any given time. You can change your target retention rate in your study settings.
Lowering the retention rate can have a big effect in how many reviews are due each day, but it will probably take weeks to change, depending on how long you've been Skrittering. Only reviews done after the change will be affected. So when you change the rate, give it time. It's often more efficient in terms of items-learned-per-minute-spent to target a retention rate lower than 95%, but forgetting more prompts is more frustrating to most people. Raising it to 97% will eventually give you a very high review load, but you will rarely forget, as long as you're practicing regularly.