There are no arbitrary mathematics. It is something called a linear model.
In high school, you probably learned about how ax+b = y, which is the formula for a slope. a and b are constants. This formula however also models a constant rate of change.
For a simple and less personal example, you can use this to model how long it takes to fill a swimming pool. Let's say that you add 10 gallons a minute to the pool, and the pool is an Olympic size swimming pool, with 660,000 gallons. You can use this simple linear model to figure out how many minutes it will take to fill the pool.
The mathematics here is
10 * x + 0 = 660000;
To solve for x, it would be 660,000 divided by 10 which is 66,000 minutes (45 days, so you're going to want a bit of a faster pump).
Similarly, the model I produced is very similar. It is not 100% accurate, but I lined out possible important factors that would likely adjust the actual value considerably. I made the following assumptions to ensure that a simple linear model could be used:
*That the rate of change is steady, and is the total number of victimizations a year in 2008.
*That the base population at the beginning is 160,000,000, or half of the current US population.
*That other factors will not affect these issues.
*That no individuals are raped twice.
The idea behind the model is very simple. With the base population of 160,000,000, we model tracking their lives. Each year, 200,000 of these individuals become victims of rape. For the sake of the model, these victims are mutually exclusive (in reality they aren't but that actually inflates the value).
Our model is mathematically represented as such:
200000 * x + 0 = y;
Because we are adding 200,000 rape victims every year, the ratio of rape victims to the total population increases every year. The first year, this ratio is 1 victim per 800, then it becomes 1 in 400, and as it goes on and on, this ratio increases.
Since we want to know how many years it takes to have the "1 in 6" statistic, we want to figure out how long it takes when the number of victims equals a sixth of the population, which is approximately 27,000,000. Now that we have a y value, we can determine how long it takes to reach that value, at a constant rate of 200,000 rapes per year.
200000 * x + 0 = 27000000;
Solving for y would be 270000000 divided by 200000 which is 137. You can perform this on your computer's calculator as a check. This means that, at a rate of 200,000 rapes a year, it would take 137 years for the population of 160,000,000 individuals to reach a ratio of 1 in 6, which implies 27,000,000 victims. No human has ever lived 137 years. It is logical to therefore to conclude that the 1 in 6 statistic is too high.
We can solve for the value based on how many years they lived by simply setting x to 80.
200000 * 80 + 0 = y;
y would equal 16,000,000; when this value is compared to the total population, it is exactly 1 in 10.
Since the value includes victims of sexual assault and attempted rape, if we use the actual value for recorded rapes, which is 85,100, the calculation becomes
85100 * 80 + 0 = y
y would equal 6,808,000, which is about 1 in 23.5 of the female population.
The other statistics regarding assault and robbery were calculated similarly.
I cannot explain this any simpler, if you do not understand I highly recommend you study up on algebra, mathematical models and statistics. If you cannot understand this you really do not have any idea what you're talking about. It's not an insult, frankly, I think most people aren't aware of their own mistakes in statistical interpretation, since most people don't understand what statistics mean or aren't good at applying mathematical models.