How can trailing and forward P/E ratio of main stock markets show such huge difference I've recently found this web page, listing the average P/E Ratios of main stock markets:


http://www.zutosmoney.com/


Here I reproduce just a small extract:


INDEX - Country - Trailing PER - Forward PER
CAC40 - France - 24.3 - 13.2
IBEX35 - Spain - 21.4 - 12.7
NASDAQ - US - 34.5 - 18


I've noticed two things:


Most trailing P/E ratios are quite high: I understand that 'normal' values should be in the range of 16 - 20.
There is a very big difference between trailing P/R and forward P/R. I understand that trailing is calculated using the actual earnings of past year, and forward is calculated with the companies' annual forecasts. 


Am I right concluding that:


Stock markets are probably over-evaluated, considering the earnings in 2016?
Stock markets are possibly over-optimistic regarding the earnings in 2017?


[Edit: I've corrected my conclusions after answer from Klas Lindbäck, to avoid expressing too strongly my personal concerns about the earnings of companies in 2017]
Am I right concluding that:

Stock markets are probably over-evaluated, considering the earnings in 2016?
Stock markets are over-optimistic regarding the earnings in 2017?


No, you cannot draw that conclusion a priori. You can conclude that the market expects a big increase in profits in 2017. You can have an opinion about whether the market is over-optimistic, but unless you got divine insight, or have better models than they do, you cannot conclude that their forecast is wrong. Remember that the market is a collection of people with access to roughly the same information that you have access to.