Whenever you produce more than you need, you are creating waste. One typical symptom for over-production is inventory waste. One type of waste which goes hand in hand with over-production is over-processing. I will discuss this later.
Over-production is making things which are not needed.
Do you cook at home? Then you may know the situation that after a meal, a lot of food is left over. You have the option to throw it away or put it into the fridge or freezer. At work, producing a lot of long reports instead of a single, short, and crisp report is a good example. If nobody reads such reports, you have created waste. Well, sometimes legal requirements may force you to document a lot which is never read. So be careful, in that situation you could argue that being legally correct is a value.
Writing long and detailed process description which nobody will use is over-production. I continuously ask myself whether the details I provide about Lean Self are too much (over-production) or just the right amount (value adding).
For companies, typical over-production is to build more than can be sold in the market.
Another simple example: you want to invite to a party and prepare a lot more invitation cards than required.
Excuse me for not giving you more or better examples. Over-production is one of the wastes that occurs rather seldom in my own life. I assume I have a natural aversion against it.
The damage of over-production usually is that you need to throw things away into which you put a lot of work. Over-production goes hand in hand with waste of intellect, inventory waste, and sunk cost.
How do you measure over-production?
The best way to measure over-production is indirectly. You can measure the inventory waste created by the results not needed. Alternatively, measure waiting waste: how long does it take until the over-produced inventory is removed? Finally, you can count how many of the results you (should) throw away.
 With one exception: I write long texts with far too long sentences, as you may have discovered already…