Stiffness is very tough to quantify since a ski’s flex pattern can vary greatly from tip-to-tail and those small intricacies will make a huge difference in ski behavior.
The reality, however, is you don't need to know what “the tip is a little stiffer” or the “tail is soft” actually means. Any good ski is well balanced and a ski engineer knows how to keep people feeling good.
Instead, the only thing you need to know is that a stiffer ski needs more energy to bend and will reward you for that extra effort in turns and control, while a softer ski, while easier to bend, won’t give quite the same energy back.
What you give is what you get.
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FACTORS TO CONSIDER:
There are 5 things you need to consider when picking out a ski. Remember, these are aspects you need to decide - not your buddy who tells you stuff. Sure, take it into consideration, but remember you are buying for you, not them.
FACTORS TO NOT CONSIDER
Just as important are the things you SHOULD NOT consider when picking a ski. In my experience, with all the things to think about, I would worry about these the least.