Our church has 1,000 members!

Measuring numbers of the current moment alone (a snapshot) is a vanity metric: any number looks good, but the number alone tells you little. Often, the number is just used to make you feel or look good. Two numbers over time can give you growth rates, which show the reality of the situation. Our church had 500 members in 2005, and 1,000 in 2015–we doubled in a decade. 

To calculate the exponential growth rate, the formula is ((Present/Past)^(1/(years))-1. Or, in the above example, ((1000/500)^(1/10))-1, or 0.071, or 7.1%. The rule of 72 (72/AGR, or 72/7.1, or 10.1, tells you how long it will take to double again at the present growth rate.

Numbers without growth rates can hide the reality of stagnation. Growth rates without numbers can hide the reality of small numbers. 

We grew by 100% per year! (We went from 2 members to 4.)

Measuring numbers against a goal gives you scope of task remaining. Our church has 1,000 members, and there are 100,000 people in our city. 99% of the task remains. It took us 20 years to get to 1,000 members. At that rate, it would take 20 * 99 or ~1,980 years to get to the whole. 

Measuring current position against position last year and against goal in the future gives you strategic critique: Do we want to take two millennia to see our city largely Christianized?  Anything that isn’t (and, most likely, can’t be) measured against a goal: I suspect it’s a “vanity” metric meant to look good on a report.