Add methods from ACS General Handbook Ch 8

[nkrishnaswami]

Add methods for estimating MOE for

• proportions,
• ratios, and
• products

[palewire]

Thank you. I think we will ultimately accept all of these additions to the library. I just want to brush up my own understanding of the math, expand the documentation a little and strengthen the unittests.

I've pulled down your PR to a branch and pushed some incremental updates to the repo. They don't show up here in the PR but maybe we can use this space to walk through them together.

The proportion documentation in the ACS handbook uses the single female population in suburban Virginia as its example. I'd like to add those values to your unittest. I've done so here, but I suspect I'm misunderstanding what the expected result should be. What am I missing?

[palewire]

I'm also curious about the reason for multiplying by 1.0 here. Doesn't the result come out the same regardless?

[nkrishnaswami]

There is a missing squaring of a denominator for ratio and proportions. I've updated that, and the testcases to check the examples in the Handbook.

[palewire]

Thanks for the updates.

How would you feel about substituting from __future__ import division to the top of the script to provide reverse compatibility on division of integers?

[palewire]

As far as the proportion formula, am I correct that a synonymous implementation would be something like:

squared_proportion_moe = numerator_moe**2 - (proportion_estimate**2 * denominator_moe**2)
proportion_moe = (1.0 / denominator_estimate) * math.sqrt(squared_proportion_moe)

[palewire]

Educate me. What is the functional difference between the proportion and the ratio code? The plus instead of the minus?

[palewire]

I've also noticed a percent change formula in the handbook. You think we should considering adding that too, or not?

[nkrishnaswami]

Whoops, missed the percent change section. I'd be happy to make another PR for that.

[nkrishnaswami]

Re proportion vs ratio, the first order Taylor approximation for variance of a function independent random variables X and Y (so we can ignore the covariance term) is I haven't chugged through the algebra, but I expect the sign difference between ratio and proportion comes from whether f(X,Y) is or