A friend of mine recently got into a discussion on a tennis forum about "what exactly is an unforced error".

You're probably familiar with the term unforced errors in a tennis game.

Unforced errors are those types of mistakes that are supposedly not forced by good shots of an opponent.

You'll often see the stats in official matches that include first-serve percentage, aces, winners, unforced errors, and so on. (below the match stats of Serena v Dinara - Serena 7 Dinara 21)

The person recording the statistics tries to determine if, when a player makes a mistake, that mistake was forced or not forced by a good play by his or her opponent. To make the right call, the statistician looks primarily at whether the player who made the error is under time pressure or is moving during the shot execution. If one of those factors is true, then that player was forced into a difficult situation by a good shot and thus made a forced error.

But if the player seems to have balanced himself or herself for the shot and to have enough time to execute a normal stroke (not abbreviated one) and still makes a mistake, that would count as an unforced error. As you can see, the definition of an unforced error is purely technical.

There is no mention of physical fatigue, tactical decision making, and mental game.

There is also no mention of the difficulty of the tennis game.

Imagine a beginning tennis player: every ball that comes across the net is difficult and is forcing that player into a difficult situation. The player does not yet have the skills needed to hit the ball into the opposite court and has to improve many tennis skills in order to rally comfortably.

As a result, we really cannot talk about unforced errors for players who have not played enough tennis. Eventually, the technique and the skills improve and that tennis player is able to rally consistently in a cooperative situation with a partner.

Almost every tennis coach talks about consistence and not perfection. It would be unreasonable for coaches to demand that beginning players make zero mistakes. However, the goal for more advanced players is to get better and better, but perfection is unattainable for imperfect human beings.

To understand unforced errors better, imagine you consistently hit balls (or makes shots) in practice in various situations where you are not forced into a difficult playing situation. These would then be neutral and offensive situations. (If you are in the defense mode and make a mistake, it will count as a forced error.) Consistently, in this context, means with a high percentage: more than 80% for offensive shots and more than 90% for typical neutralizing rally shots.

So what causes (forces?) those unforced errors?

You views please.