After reading this, you’re 250% more likely to understand odds ratios

By aussiegall from sydney, Australia (30 Days of Gratitude- Day 15) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia CommonsOdds ratios give researchers eye-catching (and media-catching) results. They also rely on readers’ confusion about what they mean. They’re becoming fashionable in research and in media precisely because they are so eye-catching, but don’t be fooled by a weak finding dressed up to look like a strong one. Here’s a quick primer on how odds ratios work.

Let’s say I publish a book. Just for fun, let’s call the book America’s Cupcake Scourge.

Read more

New documentary tackles the divorce industry

DivorceCorp opens in January. It looks great — with one little caveat.

Image from the DivorceCorp web site, www.divorcecorp.com

Divorce is big business. Many people can have a hand in the divorce process: lawyers, mediators, custody evaluators, therapists, court systems, and others all say they want to help divorcing couples. And all want to be paid for their services. The entire system can suffer from what might rightly be called perverse incentives — strong pulls for people to act in ways that are more out of self-interest than the true long-term best interest of the couple they claim to be trying to help.

This is the thrust of the documentary DivorceCorp, opening in major cities January 10. The movie looks good and important. Dr. Drew narrates, and it features interviews with some well-known law experts, including Gloria Allred. Here’s the trailer:

More information on the film, including local theaters showing the film when it opens, can be found on the official DivorceCorp website.

One cautionary note, though: The opening statement in the trailer, “50% of all US marriages end in divorce,” is wrong. As you can read about in more detail over at the excellent DivorceSource web site, the US divorce rate probably never topped 41% and has been declining for several years. As Tara Parker-Pope documented quite well in her book For Better, divorce rates are especially low among those with at least a college education. Over Twitter, the film’s reps have said that there were bigger fish to fry, so to speak. I get that. They’re looking at an entire divorce industry, and my caution is with one statistic. I believe the social conversation about the divorce rate is one specific part of the larger social conversation about divorce that especially needs to change, for reasons I’ll save for a separate post, but don’t let that take you away from the big picture. I’m happy to support the film and eager to see it.

# # #

Your comments are welcome. You can post them in the comments below, by email to ben[at]bencaldwell[dot]com, or on my Twitter feed.

How to get the divorce rate wrong

Do what a CNBC reporter did: Ask divorce lawyers.
                                                                                                                                                                                                                                                                                                           

Wedding ringsI’ve mentioned here in the past that estimates of the divorce rate are notoriously difficult to make well. There’s some inherent guesswork involved, unless you just wait for an entire annual cohort of marriages to reach either death or divorce. And that would take a long time. To simply compare a given year’s number of marriages with that year’s number of divorces is to compare different cohorts, making estimates of the divorce rate done that way wildly inaccurate.

Instead, demographers and social scientists do the best-educated guesswork they can based on past data and current trends. (Government data does not do forward-looking prediction, but rather focuses on divorces that have already occurred.) As new divorce-rate studies are released, you can keep up with them on the Divorce Statistics and Studies Blog. Reasonable people can disagree about the best scientific ways to determine the divorce rate, and there is probably some value to several different approaches. So news reporters, talking to scientists, will sometimes come up with different numbers, and that’s okay. They tend to wind up in that least the same neighborhood. (That neighborhood, by the way, projects the divorce rate for people getting married this year in the low 40s, percentage-wise.)

What is not okay — what shows rather extreme laziness in news reporting — is to ask a scientific question of someone who is in no place to answer it, and then not bother to check the accuracy of their statement. What’s even worse is when that person can directly benefit from providing misinformation.

So it went with CNBC.com in September, when reporter Cindy Perman opened her story about affairs (reprinted by USA Today) by providing an estimate of the divorce rate — a measurable, objective, scientific thing — helpfully volunteered by the director of the American Academy of Matrimonial Lawyers.

The problems with this are so obvious that I’m stunned the article was printed. I don’t even blame the lawyer, at least not any more than I blame spokespeople from the National Association of Realtors for suggesting that any economic news, good or bad, means it’s a great time to buy a home. They’re lying, but that’s their job. I just wish reporters would subject those claims to actual scrutiny.

Like, fact-checking.

I’ll say it here again: The divorce rate in the US probably never got as high as 50%, and is currently declining. The best current estimates of the divorce rate place it in the low 40s, and the divorce rate is much lower for well-educated couples than for less-educated couples.

It’s a topic taken on quite well by Tara Parker-Pope in her book For Better, which dissects the science surrounding a number of elements of marriage and divorce.

Any time you see the lazy and wrong estimate that half of all marriages end in divorce, go ahead and — nicely — say something to correct it. I suppose as a couples therapist I might also benefit from inflating the divorce rate, but I’d rather let facts speak for themselves — and I think effective, widely-available therapy would bring down the divorce rate even further. It would save us all some money, too.

# # #

If you know of egregious repeat offenders with the half-of-all-marriages-end-in-divorce nonsense, email examples to me at ben[at]bencaldwell.com, post a comment below, or send me a link to it on Twitter. Of course, other comments are always welcome.

Calculating the divorce rate

My Twitter (@benjamincaldwel) went all atwitter a couple of days ago with news that Russia’s divorce rate peaked in 2002 at more than 800 divorces for every 1000 marriages. Does this mean the country has a whopping 80% divorce rate?

In a word, no. While there are lots of ways to calculate the divorce rate, each of them flawed in some way, simply comparing one year’s number of divorces with the same year’s number of marriages is among the worst ways to do it. From a 2005 New York Times article on the topic, which also debunks the longstanding myth that half of US marriages end in divorce:

[Researchers note that] the people who are divorcing in any given year are not the same as those who are marrying, and that the statistic is virtually useless in understanding divorce rates. In fact, they say, studies find that the divorce rate in the United States has never reached one in every two marriages, and new research suggests that, with rates now declining, it probably never will.

Russia’s actual divorce rate is surely high, and does appear to be higher than the US. But I would ballpark Russia’s divorce rate in the 50-60% range for couples getting married this year. I’ll explain how I got there momentarily.

The difficulty in calculating the divorce rate is that it necessarily involves speculation. The only way to avoid some kind of speculating would be if you took a sample of people married in the same year (1930, let’s say) and then waited for everyone married that year to have their marriages end, either through death or divorce. You’d get a pretty accurate rate that way, you would just have to wait a long time to get it.

Most of us are interested in more current numbers. For example, how many of the couples getting married this year (or at least, some year in the not-too-distant past) can we expect to see end their marriages in divorce? Since not all of those couples have divorced yet, we’re into speculative territory.

What some researchers do is what they call a “life table” approach. They may take an estimate of the time frame in which half of divorces occur — usually between 0 and about 7 years of marriage, though the exact timeframe varies — and work from there. For example, let’s say we have a sample of 100 US couples married in 2002, and of those 21 have divorced as of today. If we have reason to believe that half of the couples married in 2002 who will divorce have now done so, we could estimate the divorce rate for couples married in 2002 to be around 42%. But that is an educated guess. We don’t really know where that midway point of divorces takes place, we just guess based on past history and currently identifiable trends. And if we want really current numbers, like an estimate for couples who are only now getting married, we need to be even more speculative: We now must also guess about where that midpoint in the distribution of divorces is going to be. Such an approach, even when done as well as possible at the time, runs an inherent risk of making you look foolish in retrospect.

Further complicating matters, divorce rates are influenced by a variety of factors, some more easily predictible than others. Demographic, economic, and other social changes are just three examples, and all of these have had major impacts on Russia in the past two decades.

Ultimately, no one knows precisely what the divorce rate for couples marrying this year will be, because it’s all based on speculation about possible future divorces that haven’t happened yet.

Now, back to Russia. As I said at the beginning of this post, even though they can document 800 or more divorces per 1000 marriages in 2002, that is absolutely, unequivocably not an 80% divorce rate. It is an inappropriate comparison of two very different cohorts. The population is different. That same article on Russia notes that from 1980 to 2005, the number of marriages decreased by one-fourth. So, relative to the population of couples available to be married, the population of couples available to be divorced has been growing significantly.

For the purposes of demonstration, I’m going to make up some numbers to show how flawed it is to compare a given year’s number of marriages with that same year’s number of divorces to try to calculate a divorce rate.

First, let’s assume an actual divorce rate of 60%, and that the rate breaks down to 10% per year in the 1st through 6th years of marriage. (This is wrong, of course, but used for demonstration purposes.) Second, let’s assume that the marriage rate declined 25% from 1988 to 2002, and declined evenly in that time. This is a small exaggeration, since the actual 1/4 decline took from 1980 to 2005. Finally, let’s assume the actual divorce rate had held constant at 60% since the dawn of time. If we use 1333.3 marriages in 1988 as our starting number (you can use any number there, since it’s the percentages that matter), what we would see would look something like this:

1995
Marriages: 1167
Divorces: 750
Apparent divorce rate: 64.29%
Actual divorce rate: 60%
Difference: 4.29%

2002
Marriages: 1000
Divorces: 650
Apparent divorce rate: 65%
Actual divorce rate: 60%
Difference: 5.00%

Three key points here: 1, notice how this exaggerates the “apparent” divorce rate for those simply comparing the number of marriages to the number of divorces in a specific year. 2, notice how the systematic exaggeration of the “apparent” divorce rate as compared with the actual divorce rate gets worse over time. This is always going to be true when the marriage rate is in decline. 3, our process here assumed that all divorces were done by the seventh year of marriage because it makes the math easier. Making this more real by assuming that half of divorces are done by the seventh year of marriage, and the other half take place at gradually dwindling frequency rates after that, would make the systematic exaggeration of the “apparent” divorce rate much worse.

We can demonstrate this with a different set of assumptions. Let’s again assume a 60% divorce rate since the dawn of time, but now let’s assume a more realistic distribution of those divorces. Let’s say that half of divorces take place in years one through six, and the other half are evenly distributed between years 7 and 16 of marriage. Let’s assume the same marriage rate decline as what’s actually scientifically justified — 25% from 1980 to 2005. Starting from a good-for-easy-math number of 2000 marriages in 1980, we then see numbers that look like this:

1996
Marriages: 1680
Divorces: 1096
Apparent divorce rate: 65.36%
Actual divorce rate: 60%
Difference: 5.36%

2002
Marriages: 1560
Divorces: 1026
Apparent divorce rate: 65.77%
Actual divorce rate: 60%
Difference: 5.77%

These show that the “apparent” and actual divorce rates are not the same thing, and they show how comparing one year’s number of marriages with the same year’s number of divorces creates worse comparisons over time. Admittedly, they do not account for the full scope of that 800-versus-1000 number. I say the actual current rate is in the 50-60% range because (1) by presuming an end point to divorces after a specific number of years, my examples here do more to deflate the difference in actual-vs-apparent divorce rates than to inflate them; the distinction becomes greater when you model the numbers in ways that more closely approximate reality. And (2) these statistical models do not account for other social and economic changes in Russia that may be throwing off the divorce curve, that is, how divorces are distributed over time among those couples who do divorce. With those in mind, I would ballpark the expected divorce rate for couples getting married in Russia this year in the 50-to-60 percent neighborhood, though again, social and economic conditions can influence the numbers greatly in either direction.

Thankfully, there are lots of people out there who do this kind of work for a living, and can do both the detail work and the explantion better than I can. Andrew Cherlin and David Popenoe are two worth reading. More information on the flaws inherent in estimating divorce rates is here.