As I study with scritter for about 2 years I still wander about some statistic issues:
what is actually the retention rate? What is a good one? What affects the change?
As I look at the word it’s written there down “due” or “over due” in percents. I don’t get the meaning of it.
Than you a lot!
Hey there! 
:
Retention rate shows your overall success of remembering. For instance if you have done 100 reviews, and got 10 of them incorrect, that would be a 90% retention rate. Grading things as correct or incorrect is what affects the change of retention rate, and the system by default is set for a 95% ideal target retention rate.
The “dueness” of an item shows how due it is. For instance if a word were scheduled for 10 weeks after it was last reviewed, and you review it as precisely 10 weeks, it would be 100% due or “ready”. If you review that word at 11 weeks when it was due at 10, that word would be 110% due, and the grading is more heavily affected if it’s still remembered then since it’s overdue yet still remembered. In general it’s best to not overstudy (review anything that’s less than 100% due), and to try and review things on time so nothing is overdue.
Hope this helps!