Is increase of company's customer a business performance indicator?

Yes, it is an indication of your ability to attract customers, which is normally a good thing. Of course, you need to look at other performance indicators as well such as profitability, customer satisfaction and out-of-stock frequency. If you sold gasoline for a penny a gallon, you would attract a lot of customers but loose a lot of money. If you attracted a lot of customers but didn't have the staff or facilities or product to satisfy those customers, your overall performance would have been better if you attracted less customers!

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Many things are measurable. That does not make them key to the organization's success. In selecting Key Performance Indicators, it is critical to limit them to those factors that are essential to the organization reaching its goals. It is also important to keep the number of Key Performance Indicators small just to keep everyone's attention focused on achieving the same KPIs.