Economy picking means that you pick first note on the next string in the same direction as the last note you picked .
For going up the scale use this : D U D, D U D, D U D ,etc.
For going down the scale : U D U, U D U, U D U ,etc.
Strict alternate picking means that you pick every next note in the opposite direction of the last note , wether you change the string or not , and no matter what ... When you change strings , it's normal that you will face trouble if when you do the outward-picking thing , but in-ward picking should be ok.
Look at this example :
---8--
-7----
If you picked the first note with a downstroke , the second with an upstroke , it's easy to do it fast , and continuously for a period... That's "inward-picking" (the direction of picking resolves in the same direction , inwards)
But what if you picked the first note with an upstroke , the second with down ? You'll now face the "outward-picking" problem , cuz the picking direction now resolves in opposite direction , making it harder for you to play faster.
To overcome this , just do that example over and over , and practice your 3 notes/string patterns , and you'll get it in a while.
When you strict alternate pick 3 notes/string , you'll find that you'll be changing between inward & outward picking each time you change a string ... That's the reason people bother forcing themselves to practice outward picking.
