The higher your quality score is, the better. As a reminder, the quality score graph is split into 3 different sections, which helps you easily identify whether or not your data is good quality. You’ll see that above 30 is colored green, 20-30 is yellow and anything below 20 is red. 

These numbers represent the probability of error. A quality score of 10 means that there’s a 10% chance that the base is incorrect, while 40 means that there’s only a 0.01% probability of error. Remember, we're working with logs here.

Probability of error:

10: 10% 

20: 1% 

30: 0.1% 

40: .01%