“It ain’t so much the things we don’t know that get us into trouble. It’s the things we know that just ain’t so.” —  Artemus Ward 

Two years ago the Washington Post tried an experiment. The paper persuaded a violin master — Joshua Bell — to set up and play in a subway station to see if anyone would notice the world-class quality.

No one did.

It reminds me of a concert I heard years ago. The band came out and played its first song. The crowd cheered with gusto. It was only then that one of the band members said there would be a delay while he repaired a broken string. The song had been played with defective equipment! No one noticed — or cared.

The link between the subway violinist and the band with the broken string is preconceived notions.

The commuters in the subway station never expected to see a world-class musician there. And the crowd at the rock concert was predisposed to appreciate the band. Both groups of people had preconceived notions — and ACTED according to those expectations.

A throwaway song at a concert is one thing. Coaching a team based on wrong impressions is another. Preconceived notions are the enemy of every coach.

Wrong notions can lead you to pick the wrong players for your team, to overlook a contributor on the bench (the subway violinist) and to stay too long with an ineffective player (guitarist with the broken string).

Coaches must be everlastingly on the lookout for their own biases. Here are three ways to help make sure that preconceived notions don’t get in the way.

Use data whenever possible. Can you tell that a baseball player can hit just by looking at his physique and his classic swing? No. the only way you can tell if a baseball player can hit is by measuring how many hits the baseball player has.

Create a practice environment where every day players have a chance to show what they can do and where they KNOW they will have a chance to show what they can do.

Get another set of eyes whenever possible. Great executives like to keep someone around who will challenge their thinking. Coaches should compare notes often.

With all the hard work that you put in, make sure that preconceived notions don’t ruin what you’re trying to accomplish.

Coach Tully’s Extra Point: Joshua Bell created his first violin by stringing rubber bands across his dresser drawers — when he was four years old.