Tuesday, March 3, 2009

I stand corrected

I think I will eat my crow before someone dishes it up for me. I shot from the hip--or maybe from the lip. In my previous post I claimed that relative police reporting practices are responsible for the disparity in traffic crash rates between Lincoln and Omaha. In fact, this appears not to be the case. I stand corrected.

Despite differences in our reporting practices (which may still exist), the difference between our crash rates described in the Omaha World Herald article do not appear to be due to the numerator (number of crashes), but rather the denominator: million miles driven. I went to the actual source data, the monthly recapitulations of traffic crash reports from the official keeper-of-the-records: the Nebraska Department of Roads. Here's what the "winter" of 2007-2008 looks like, when you examine box 17 on these monthly reports:


It appears from these data that the crash rates per capita in these cities are very similar. I did the same math for calendar year 2007 and for 2008, with similar results: Lincoln's rate per capita is within a gnat's eyelash of Omaha's: very slightly greater in 2007, very slightly lower in 2008. This would tend to confirm Allstate's ranking of traffic crash claim rates, which put us very close together.

Since the number of crashes lines up with population, the difference in crash rates per million miles driven is attributable to the much greater miles driven in Omaha--way out of proportion to any population difference. One might reasonably conclude that drivers in a larger city drive a little further on average, but that effect should be pretty minuscule. I suspect that the theory floated by Fred Zwonechek of the Nebraska Department of Motor Vehicles concerning the influence of a interstate travel through Omaha upon the million-miles-driven denominator is correct.

In the future, Casady, do your analysis before popping off. Mea culpa.

10 comments:

Anonymous said...

At least you print your corrections at the top of your front page, which is more than anyone can say for any newspaper.

Anonymous said...

I appreciate your candor and correctness, but I still say, if you read the quote below from the World Herald, that they are counting accidents reported on days when there is snow and ice.

If you don't fill out the report, it doesn't get counted.
------------------------------
"Yet Lincoln reported 77 more actual crashes related to winter weather in 2007 than Omaha did. Lincoln drivers had 989 crashes on snow or ice in 2007, while Omaha drivers reported 912.

Milder winter weather in 2006 resulted in far fewer crashes, but the trend was similar: Lincoln reported 9.6 winter-weather crashes per 100 million miles driven, while Omaha's rate was 5.8 winter-weather crashes per 100 million miles driven.

That year, Lincoln reported 173 actual crashes, while Omaha had 229."

Anonymous said...

. . .Another reason why I appreciate our Chief. Too often the negative politics of public service gets all the press. Grateful for your gracious and humble response.

Anonymous said...

Finally admitting that you aren't Mr. Perfect.I get tired of your rants on how great you and the department are.No wonder that with your budget crunching that you will have to let some of your civilian staff go at the end of this fiscal year.Nice way to let loyal people go.

Dorothy said...

Numerators and denominators, and bears,
Oh my!

Anonymous said...

5:00, can you cite any single instance where he ever claimed to be "Mr. Perfect"? We can wait all day while you find a documented quote...no? None? Well then, I can't see how your comment make any sense at all.

p.s. Please use a space after each end-of-sentence period, thanks.

Tom Casady said...

Dorothy-

Do you think that reference is a little obscure for the readers of The Chief's Corner, or not?

Grundle King said...

RE: Dorothy

Do you mean bears like in the song 'Convoy' by C.W. McCall?

Anonymous said...

There was a guy named Al, who ran a boat ride, and was a "Friend of Dorothy". He once recited a similar ditty.

Anonymous said...

I thought Dorothy was referencing 'lions and tigers and bears oh my' but with a 'nerdy' (in a good way) twist.