**Why Quinn’s estimate is wrong**

To arrive at his 2010 estimate, Quinn starts with the last known official tithing figure, which was $78 million in 1960. Next, he calculates an annual increase of 12.9% from the known tithing figures of the preceding decade. With these two numbers, he then calculates tithing in 2010 using the following equation for exponential growth (with T standing for tithing):

Judging from the way he tabulates his way from 1960 to 2010, Quinn may not have realized he was applying an exponential function. He may have been aiming for a more intuitive linear extrapolation. Tithing in 2010, extrapolated from the actual 1950-1960 data would be:

As illustrated in the following chart:

Either way, both methods are ill-suited for the task at hand. Extrapolation is basically guesswork based on incomplete data, and exponential growth doesn’t really exist. In the real world, growth is always constrained by one thing or another.

Another issue with extrapolation is the implicit assumption that nothing changes in the underlying dynamics of the process that is being modeled (i.c. paying tithing). In this view, the Mormon Church in 2010 is exactly the same as the Church in 1960, only bigger. This is clearly not true. In fact, Quinn could not have chosen a worse data set on which to base his predictions because the growth rate of the Mormon Church as we know it started its terminal decline right around that time.

**How to arrive at a more realistic number**

To arrive at a more realistic number, it is good to realize that in the absence of the actual numbers, all calculations are approximations. Every assumption adds uncertainty to the model and inaccuracy to the outcome.

In its most basic form, the model for calculating Mormon tithing revenue is:

*Tithing = Members x Income x 10%*

There are two uncertainties in this model: how many members pay tithing, and how much? In 2015, Mormonism101.com published a quantitative model using a Monte Carlo simulation to deal with these uncertainties, and arrived at an estimate of $3.5 billion in annual tithing revenue.

For this article, the model was improved on a number of points. First, instead of putting country activity rates in 10% brackets, the activity rates of each country are now simulated individually. This reduces the skewing of results for activity rates near the upper or lower bounds of the brackets.

This in turn allows the accuracy of the activity rate estimates to be more explicitly modeled. Given that these estimates date back to 2011, the model now incorporates the odds that these estimates are too high (20%) or too low (10%).

A final improvement, suggested by Redditor u/MTC-allstar, is to present the outcome as a histogram, which shows 80% of the simulated data fall between $2 and $4.5 billion, with the most frequent outcomes between $2.1 and $3.8 billion:

**Why Quinn’s estimate is wrong - revisited**

The original article about this tithing revenue model ends with the following observation: “The result of $3.5 billion is as reliable as the estimate of the activity rates and the percentage of members paying tithing. Unfortunately, the Mormon church does not disclose this information either. What, then, is the value of this model?” and goes on to suggest that, among others, “the model can be used to validate other tithing revenue estimates”.

So let's validate. Under which conditions does the model yield Quinn’s $33.7 billion estimate?

There are two scenarios. It would either take 9.2 million Mormons to all pay 100% tithing on their country’s per capita income, or it would take all 15.9 million Mormons (men, women and children) to each pay 55% tithing on their country’s per capita income.

With only about 5 million active Mormons, and no evidence that Mormons have higher than average incomes, there simply is no plausible scenario to match Quinn’s estimate.