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.