After 50 minutes of Skritter time today, I checked my progress numbers to see that I have learned -3 characters today. How can this be?

I believe you can go negative if you get more older characters incorrect than you did new cards correct today.

Don't worry about statistics too much. Sometimes I worry too much about my graph myself. Put in more time and you'll learn more.

that might be because you added new ones and forgot older ones.

or you didnt add any new ones but remembered old ones ...

just keep buggering on as churchill said. dont worry too much about the statistics!

I suppose they could re-word it if it's a negative number from learned to forgot. Then the negative sign could also be removed.

Yeah, I figured it meant "forgot" in some sense although it surprised me since this is the first time I've seen a negative "learned" number in months of using Skritter. And, things are looking up because now I've "learned" seven characters in about ten minutes today.

By the way, how does Skritter deem a character "learned"? I have some days where the "learned" characters are higher than "added" characters, and then vice versa.

An item is "learned" once its scheduled interval goes above twelve hours, and once it goes back below twelve hours, it's unlearned. Twelve hours is an arbitrary threshold we picked a long time ago.