You can use a few pieces of data and a few math formulas to uncover some amazing trends. I will illustrate this with several posts on global trends. I will show you to do the same thing for any part of the world: a region, nation, people group or country.
Start with the population at two points in time (for example, 1990 and 2000). Get the population of a smaller part of the population. For example, find the Christian population in 1990 and 2000. Use these to figure out the non-Christian population: Total Populatino minus Christian population.
Here is the first formula: population growth rate. In a spreadsheet like Microsoft Excel, it’s written: G = (popLater / popEarlier) ^ (1/TotalTime). So, for example, you might use:
G=(pop2000/pop1990)^(1/10)
This formula can help you find out the net natural demographic growth rate. Net natural demographic growth is births, minus deaths, plus immigrants (people moving in), minus emigrants (people moving out). Next we will use this to find out the rate of conversion or defection from Christianity.
1. Compute the growth rate for the population as a whole.
G = (pop2000/pop1990)^(1/10)
The result of this formula is a ratio, not a percentage. The percentage growth can be obtained by the formula P=(G-1)*100, but that’s more for display than for use in the following formulas.
2. Compute the expected natural population for the subsect
ExpectedNatural = Christian1990*(G^10)
Christian1990 = the number of Christians in 1990;
G = The overall growth rate
10 = the number of years you are measuring for.
This gives you the expected total population of the segment in 2000, not accounting for conversion. Next, use this formula:
AnnualNatural = (ExpectedNatural-Christian1990)/10
This gives you the annual “natural” population growth.
3. Compute the actual growth rate for the subsect
AnnualTotalGrowth = (Christian2000-Christian1990)/10
This gives you the total annual growth of the segment (which would include both natural and conversion growth). This is different than the natural growth computed above.
4. Compute conversions
AnnualConverts = AnnualTotalGrowth-AnnualNaturalGrowth.
If this formula results in a negative number, then you have more defectors than converts and you’re in decline. If the number is positive, you have more converts than defectors and you’re growing (although this does not exclude the possibility of some—perhaps even many—defectors).
Here’s an example from the World Christian Database entry for Afghanistan. I’ve computed growth patterns for the period 1900 to 1970.
Afghanistan
| Religion | AD1900 | Natural | Converts | Total | Rate% | Rate | AD1970 |
| Christians | 300 | 6 | 102 | 108 | 4.81 | 1.05 | 8,020 |
| Non-Christians | 5,099,700 | 96,280 | (105) | 96,175 | 1.21 | 1.01 | 11,831,980 |
| Total population | 5,100,000 | 96,286 | - | 96,279 | 1.21 | 1.01 | 11,840,000 |
Afghanistan was marked by some small numbers of converts, but by and large the non-Christian (Muslim) population expanded primarily through births to Muslim families. The relatively small rate of conversion was enough to expand the church from just 300 people to an estimated 8,000–but this was a very small part of the overall 11.8 million.
Tomorrow, I’ll show you how to extend this with additional data into national trends projected out to 2025. Then, we’ll look at regional and global trends using this methodology.
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