Here's further data on the key elements of DELIBERATE PRACTICE: [from the “The
Making of an Expert” - Harvard Business Review, 2007, and other sources]
“0”] CONCENTRATION - ON A SPECIFIC GOAL FOR THE PRACTICE SESSION, SOME ELEMENT WHICH IS NOT ALREADY MASTERED:
“When most people practice, they focus on the things they already know how to do. "DP" is different. It entails considerable, specific, and sustained efforts to do something you can’t do well—or even at all.
"Research across domains shows that it is only by working at what you CANNOT do that you turn into the expert you want to become.”
1] AN EFFECTIVE COACH, OR ‘DRILL SERGEANT’:
“The development of expertise requires coaches who are capable of giving constructive, even painful, feedback. Real experts are extremely motivated students who seek out such feedback.
"They’re also skilled at understanding when and if a coach’s advice doesn’t work for them.”
2] DIGGING INTO MISTAKES AND CORRECTING THEM IMMEDIATELY - also known as ‘FEEDBACK’:
“The elite performers we studied knew what they were doing right and concentrated on what they were doing wrong.”
3] PRACTICING AT PEAK INTENSITY FOR SHORTER PERIODS OF TIME:
“Deliberative practice involves two kinds of learning:  improving the skills you already have and,  extending the reach and range of your skills.
The enormous concentration required to undertake these twin tasks limits the amount of time you can spend doing them.”
4] KNOWING WHEN TO SET YOUR OWN GOALS AND “PLAY” WITH THE SKILL[S] YOU HAVE ACQUIRED:
“Like good parents who encourage their children to leave the nest, good coaches help their students learn how to rely on an “inner coach.” Self-coaching can be done in any field.”
Now,  above may seem like a contradiction, but think of it as a reward for the work you put in.
For example, there’s nothing like the feeling of freedom in being able to improvise, as in Jazz.
But this can only be
achieved AFTER one has gotten his various skills down, ice-cold. And
that is gotten ONLY by practicing "deliberately".
A lot of musicians, seeing their existing exercises deliver diminishing returns, develop their OWN new exercises. These exercises can then become ‘canon’ for the musicians who follow and so the field is advanced.
And the accolades continue: David Grisman [above] uses "The Original" Cowling System:
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And still more [actual and unsolicited] rave reviews for "The Original" Cowling System:
FROM A VIOLINIST:
"The Cowling System © is a set of 12 exercises designed to increase the flexibility and strength of the hand, wrist, and fingers.
"I bought the course in 1978 and have found them to be invaluable.
"I've kept them all these years and still use them to warm up with before playing."
~ Richard Barton posted on VIOLINIST.COM
FROM A BASS PLAYER:
?"My fingers actually DO what my brain tells them to do, WITHOUT getting confused."
~ Anthony Jerome Smith, 35-year veteran bass player.
FROM A PIANIST:
"In 1976, my teacher gave me his copy of all the exercises, whereupon I made xeroxed pages to put in a binder.
"My teacher was amazed at my progress that in a year I could play, for example, the prelude #17 of Chopin!
~ "RPN58" (from a pianists' discussion board in England, 2006)
FROM A GUITARIST:
"I was introduced to the Cowling System © in the 1980's by a violinist in his seventies, who had amazing dexterity on his instrument for HIS age.
"Through the use of the COWLING SYSTEM ©, I went from "amateur" to an "A" level in music in a very short number of years.
"I have been using the exercises continuously since my friend gave them to me and believe me - THEY WORK."
~ Don Wills Guitarist, Great Britain
FROM A VIOLINIST:
"I must say I am highly delighted with the way my fingering has improved since I commenced your hand strengthening exercises.
"Recently, I became very depressed about my playing and almost decided to give it up, but now I feel it's a pleasure, thanks to the Cowling System. ©"
~ "THE VIOLINIST" Magazine, December, 1924
I'd love to see YOUR testimonial on this list! If you really do "THE ORIGINAL" COWLING SYSTEM © of Hand Strengthening Exercises", you CAN master and truly enjoy your music!