The Due Date Margin of Error

2008 US gestation distributionWhether you want to schedule a last fling before the baby comes, or this is your nth kid and you need someone to babysit the other n – 1, knowing the margin of error on your due date would make planning a lot easier.

This knowledge can also give dads credibility in certain parenting circles and in baby-related decision making, which may help boost their confidence and moral.

Below are some US margins of error and a few other handy statistics that will quickly prove you are an engaged and caring father. Warning: also included below are some intimidating pregnancy words (e.g., gestation, menstrual, conception). Apologies for any anxiety they may cause.

Information and Sources

It’s hard for a dad to get good information when something like 98.6% of statistics are made up on the spot. Online are a variety of due date confidence intervals, from 12 days, to 18 days, to 4 weeks; so there’s a large margin of error on the margins of error. Also, someone said that 60% of people don’t credit their sources.

Tired of the confusion and in need of a moral boost, as we’re approaching n = 4, I decided to go to a pretty reliable source – the CDC birth data warehouse. The stats below are based on the latest CDC data set, a 3.2 GB file containing natality information on the 4.26 million births registered in the US in 2008.

Some Background

First, some background info that I recently learned. At the start of the pregnancy you have a due date – when the baby is most likely to be born, based on when the doctor estimates the baby was conceived. Once the baby arrives, you can translate this date into an estimate of gestation, the number of days or weeks prego, by counting back to the estimated date of conception.

Stay with me.

The standard initial due date estimate is 280 days from the last menstrual period (LMP; ask your wife if you’re confused), or about 266 days from conception. This estimate is sometimes adjusted based on ultrasounds and such, but it’s typically close to 40/38 weeks.

So – we’re interested in the average gestation for women in the US and the variability around that average. The average, 38 weeks, is what they tell us at the early prenatal visits. The variability is key, as it will tell us the likelihood of the 38 for a randomly selected case, i.e, for our next kid.

The Stats

The plot above shows the adjusted gestation distribution for single births, in percentages. And the table below contains the mean, standard deviation, skewness, kurtosis, and count, for the adjusted and LMP gestations in 2008. I removed extreme outliers, flagged records, and missing data, which brought the counts down to under 4 million. Note that the adjusted estimates are much more accurate.

Some Gestation Stats
Estimate Mean SD Skew Kurtosis N
Adjusted 38.59 1.94 -3.21 22.61 3665402
LMP 38.73 2.37 -1.77 12.82 3672562

The take home message: due date margins are like the big stretchy waist bands on maternity pants. Only 75% of expectant moms have their baby within 10 days of the standard due date; 85% fall between weeks 37 and 40, making a huge 28-day margin. Plan accordingly.

By the way: 4.26 million births? That’s 8 per minute!

Taking a Psychology Lesson From Our Kids

Care bear costumeOur kids are novice humans. We’ve been around the block like 30 times, and the only block they know is for building towers. Yet somehow they’re experts at manipulating us to get what they want. The worst part is, we encourage it, without knowing it.

Consider this common scenario involving a dad and his 3-year-old at the playground:

Kid, in his most dramatic whiny voice: Dad, can I go to Aladdin’s house for lunch? Please, please?!

Dad: Hmm… probably not today, since lunch was 6 hours ago and we’ve already had dinner.

[Kid produces an amazing fit of screaming and thrashing]

Dad: Buddy, I know you’re sad, but it’s not OK to act like that.

[Kid goes full bore into hypertantrum mode, which resembles a cross between the Tasmanian devil and Jack-Jack from the Incredibles]

Dad, embarrassed and worried his kid is going to pop a blood vessel and permanently damage vocal chords: Hey, if you calm down, maybe Aladdin can come hiking with us tomorrow. Otherwise, we’re going home right now.

Phew – disaster averted. Nice job dad!

What have we done?

Little kids are still learning to manage their emotions, so we should reward them when they’re able to calm themselves, right?

The problem is, in these situations we aren’t teaching our kids to manage their emotions. Instead, we’re teaching them to utilize their emotions to get what they want. Our kids have trained us to give them something, to reward them, in exchange for immediate cessation of all sobbing, whining, tantrums, and general misbehaving.

It seems like the hiking trip is a reward for potentially good behavior – that would be positive reinforcement of something we want our kid to do, manage their emotions.

But in this case the kid is in charge – they started the exchange by punishing us, with something that can induce a headache and cause grouchy-dad syndrome, while implicitly offering to stop under certain conditions. That’s negative reinforcement – taking away something bad so as to reward and encourage someone’s behavior.

What should we do?

In my opinion, the tantrum itself needs to have consequences, not the ending of the tantrum. If they freak out while doing something fun, all the fun should end. Ideally, we’ll have told them ahead of time what the conditions are for being at the playground or on the campout.

Sometimes they forget, and maybe we give them a second chance. Not a big deal. But we shouldn’t offer special rewards to get them to stop doing something wrong.

Scoring Gear and Other Deals on Craigslist

craigslist rss smallWhether you need the services of a Jedi master or you’re looking to rent igloo space, be the first to know when the posting hits Craig’s list with an RSS feed. RSS makes it easier to skip past the wire transfer scammers, people selling things like gently used cloth diapers, or people whose stuff is “reel cheep,” and get to the quality bargains.

Once you’re happy with your search terms, e.g., “Kelty kid carrier,” zoom down to the lower right corner of the page to get the feed link. To subscribe manually, copy the link and take it to your RSS reading software, e.g., Thunderbird or Google Reader. Or click it, for instructions on subscribing from your browser.

You’re reader will grab any posting that matches your search terms as soon as it’s created.

craigslist rss

More info on RSS from craigslist, and Wikipedia.

Being a Good Parent: Having Kid Cred

dad on dirt bike

Though I still feel and act like a child, I guess I would consider myself a dad. My kids are going on 0, 2, 4, and 6, so that gives me like 12 combined years of experience raising infants and toddlers. I can change a mean diaper. I can also impress my kids by jumping over a fire hydrant. But as they get older I’ll have to think of newer, better ways to convince them that I’m cool, and worth listening to.

In high school I taught gymnastics to preschoolers every afternoon. As with my own kids, I could get their attention and get them to try a new trick by doing it myself and making some kind of rocket laser-beam noise as I went. Then, I started working with 6-year-olds. To keep them from poking each other to death, especially toward the end of class, I had to do an occasional back-flip.

Finally, my senior year I taught 12-yr-olds, boys, who didn’t actually want to be there. No trick was cool enough. But, I think they learned to listen and to trust me when I said things like “keep trying, you can do it,” or “leave him alone or I’ll pull your ear off,” once I told them about my dirtbike wrecks and recent paintball battles. Once I established my kid cred, I wasn’t just their teacher or coach, I was in the club.

Unfortunately, parents and kids often establish separate clubs – the old fogies and the little whippersnappers, each having its own definition of what’s cool. So, we never really understand our parents, and what they did for us, until we put on their shoes – the parent shoes. In a similar way, we might not understand how cool they were, looking back, until later, when we redefine the term to include “makes ridiculous sacrifices for ungrateful brats.” But this isn’t about appearing cool after the fact, as important as that may be. It’s about being cool during the fact, and getting in the kid club while it still exists. Actually, it’s about my dad, and the outdoors.

The outdoors bring the fogies and the whippersnappers together like nothing else can. With dirt, sticks, rocks, bugs, and creeks the activities and the adventures are unlimited. As kids get older, they might need some encouragement and some structure, some equipment or something motorized. But, regardless of age, playing outside is the coolest of family activities – there’s no better context for establishing our kid cred. We’ve got to find something that our kids think is cool, learn to do it, and share it with them.

And so, with father’s day approaching, I want to highlight, and thank, my dad, for mastering this principle of parenting – for our monthly trips to the beach, or into the Sierra Nevadas, hiking, camping, and exploring; for road trips to Yellowstone, Banff, Glacier, Zion’s, the Grand Canyon, and everywhere in between; for teaching me to dirtbike and for still being faster and getting more air; and, most importantly, for being someone I’ve always looked up to. You’ll always have cred on my street.

How Bad Would Another Kid Be?

new kid difficulty functionOccasionally people ask us what it’s like to go from n to n + 1 kids. The occasion is a couple, with one fewer kid than us, considering having another, which they’re afraid may be impractical and illogical, maybe even irresponsible. The simple answer is, it’s not as bad as going from n – 1 to n.

Mathematically, the more kids you have the easier it is to increment because you’re adding a difficulty factor of 1/n. From 0 to 1 you’re increasing by an impossible, infinite amount, 1/0; from 1 to 2, by 100%; from 2 to 3, by 50%; etc. So number 1 is a dramatic shock, but the proportional increase gets smaller the more you add.

That’s what people like to hear. It makes sense, and it sounds feasible. Keep in mind, the main assumption of this model is that your next kid will be of similar difficulty. Instead, kids usually differ in temperament, where one is an angel and the next is their alter ego or, worse, their arch nemesis. Just when you’ve got things under control with n kids, changing diapers one-handed, the nth + 1 shows up and you’re changing diapers no-handed. Chaos. Anarchy. Then you figure it out again. Then you have to potty train, again.

So, overall, in the long run, on average, the next kid is easier than the last. If you can make it to n = 6 you’ll hardly notice the change.