I read the “How to Avoid Getting Lost in the Numbers (Harvard Business School HBS Note)” again. (as mentioned in my previous blog "Number is meaningless unless compare with something" )

You can always discover "new" ideas while you reading some "old" materials.

I found 2 "new" things:
(1) Percentage: Be sure you know and record what you are taking percentages of. That is, when you calculate A/B = C%, always check what's your "B". In an exam, if you score 10/100  = 10% (for full mark 100), but if you score 10/10 = 100% (for full mark 10).

(2) Average: (500 + 1500)/2 = 1000. (999+1001)/2 also = 1000. It's important to know the spread. If Company A paid dividend, year 1 = $3, year 2 = $1; Company B paid dividend, year 1 = $1.9, year 2 = $2.1. Usually Company B is preferable because investor like stable grow....