Andrew Bak wrote:Why 16 games? Why not 10, or 30? Is this an arbitrary choice or is there some underlying mathematical reason behind it?

30 was chosen because there is a 95% confidence in the grade being correct. Further explanation can be found here:

http://en.wikipedia.org/wiki/Confidence_interval
10 is another confidence interval, but I can't remember off-hand exactly what it is. It was reduced to 9 because there was enough demand for it to be reduced to 9. The various grading categories correspond with other confidence intervals, but I can't remember what they are.

The maths behind this is sound in theory: Because you calculate a grade by adding stuff up and dividing it by a number, i.e. you find the mean, the more games you play, the more accurate your grade is.

For example:

If I have 900 points over 9 games, I have a grade of 900/9 = 100. If I then beat a 100 in the last game in the period, my grade goes up to 1050/10 = 105

If I have 9000 points over 90 games, I have a grade of 9000/90 = 100. If I then beat a 100 in the last game in the period, my grade goes up to 9150/91 = 101, assuming you round up to the nearest whole number.

The result of one game makes less difference to your grade, so the figure of 100 (or 101) is far more accurate, so you can say statistically that you have more confidence in its accuracy.