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In his 1971 book Wheels, Arthur Hailey claims that cars that were primarily assembled on a Monday or Friday would suffer from quality problems due to worker performance/absenteeism issues associated with those days. Now, I realize that this was a work of fiction, but as an origin to this becoming a widely held belief, there are folks at Snopes curious about it (and repeat they have heard this from some un-named source), as well as other random internet forums discussing this "fact". And in 1979, Vance Muse wrote a book called "Don't buy a car made on Monday" which appears to be an advice book!

So, while the origin seems to be a work of fiction, is there any truth to this widely-held belief?

Oddthinking
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Larian LeQuella
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    I have a good idea what the answer is, but I'll leave it for the community to answer this so I'm not taking away the opportunity for someone to wow me. :) – Larian LeQuella Jun 17 '11 at 21:05
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    Where is the "born on date" on a car? – Hendy Jun 17 '11 at 21:20
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    Yep, that was one of my very first counters in the discussion I was having. There are more. :) – Larian LeQuella Jun 17 '11 at 21:22
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    Cars are not completely assembled all in one day. The final build attaches all the subassemblies together, but the various parts are all built at different times. So. Which date are you interested in? Further, if a vehicle is half assembled on Friday, and half assembled on Monday is that twice as bad? – Adam Davis Jun 18 '11 at 00:01
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    @Adam Davis, I settled this question during my discussion that I had with a less skeptical friend. I am putting this question out here because it appears to be a "somewhat" widely held belief. One of the best ways for a person to remember an answer is for them to do their own research. If you have a nicely referenced answer, feel free to add it, and you can get the +1 and even an answer select from and other users here. :) And have fun too. – Larian LeQuella Jun 18 '11 at 00:09
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    Unfortunately for you I'm pretty lazy. I'd approach it from the perspective of the cmmi and iso standards and processes they use to ensure that all processes are performed exactly the same for every vehicle regardless of time or assembler. I suspect there may be follow up studies since the seventies, but since they don't show poor quality tied to date, they won't be widely published. Further the problem I mention above is even more true today as most subassemblies are now outsourced to suppliers, whereas in the seventies manufacturers were still largely vertical. – Adam Davis Jun 18 '11 at 00:46
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    @Adam, all great stuff. Actually, Google helped me with an old Popular Mechanics article that directly addressed this too. – Larian LeQuella Jun 18 '11 at 03:02
  • @Larian - donno about American cars, but as far as Soviet anything (not just cars) it's fully 100% true as far as quality of work (with obvious adjustments to things that take >1 day to work on). ISO standards aren't translatable into Soviet :) – user5341 Jun 18 '11 at 16:20
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    @Larian - ditto for a lot of American software (from personal experience :))))) – user5341 Jun 18 '11 at 16:22
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    There is a word for this is in swedish "måndagsexemplar", so it seems the idea is at least widespread. :) – Paula Jun 29 '11 at 08:29
  • Are cars even built on a Monday-Friday schedule? I thought everyone used a shift pattern? – DJClayworth Aug 24 '11 at 16:52
  • come on you expect me to except your workplace safety report as equal to if a car is built well or not on a Monday or Friday? You ever heard of the term pivot? in addition I would like to point out that as somebody who has spent a great deal of time in the German automotive business cars are built in one day were in the year 2013 may be you or member gentleman called Henry Ford who invented the modern assembly-line or do you think that those cars were produced in 15 to 25 days? You need to take a trip to to Munich visit Bavarian motor works better known as BMW and watch as their cars are built –  Apr 16 '13 at 01:03
  • This claim sounds legit. If you think about it, Monday's you are recovering from the weekend, Friday's you are rushing for the day to end. Both these days, your work ethic and quality is less than the other days IMO. Or maybe it is just me :) – NuWin Sep 15 '15 at 03:46
  • This reminds me of a recent BBC headline: "Are you more likely to die in Hospital on a weekend?" – PCARR Sep 15 '15 at 16:54
  • In USSR there was a belief that everything made in the end of the year is of much lower quality - as they was "making the plan", not the product. – Eugene Petrov Sep 15 '15 at 23:30
  • I would like to add that nowadays most car manufacture plans work 24/7 making those assumptions irrelevant, as employee that work on Saturday night shift may have his "weekend" on other days. And employee that assembles a car on Monday morning shift could possibly worked day before (Saturday) or had his weekend. – Ilya Lifman Jul 12 '17 at 12:50
  • Regardless of accuracy, I want someone to go in and demand a car that is certified to not be made on a certain day of the week, make the completion of the sale contingent upon said certification, and report back on car salesperson contortions observed. – PoloHoleSet Jul 12 '17 at 15:54

1 Answers1

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While it's obvious, as @Adam Davis points out, that complex products, like cars, are not manufactured in a single day, it may still be of interest when critical parts of the car, such as safety belts, were installed, if Mondays and Fridays are bad days for manufacturing.

Statistics for manufacturing quality per weekday are hard to come by. However, if low quality manufacturing is assumed to be caused by overall lack of concentration (due to still-drunkenness on Monday, or due to thinking about drunkenness on Friday), then I would argue that manufacturing errors leading to workplace accidents should follow a similar pattern as manufacturing errors leading merely to low quality products.

Ruckart and Burgess (2003): Human error and time of occurrence in hazardous material events in mining and manufacturing. Journal of Hazardous Materials, 142:747 who looked at the distribution of accidents reported to the "Hazardous Substances Emergency Events Surveillance (HSEES) system maintained by the Agency for Toxic Substances and Disease Registry (ATSDR)", found that

Most events (2781 [84.7%]) occurred on a weekday, with the greatest number of events occurring during Tuesday through Thursday (619 [18.9%], 565 [17.2%], and 587 [17.9%], respectively).

Which means that on average 17.0% of accidents occur on Mondays or Fridays, respectively.

There is a very similar trend in construction, using data from the Occupational Health and Safety Agency (OHSA), as shown in the Master thesis by Ashwini Bhide from Ohio State University (p. 33). Note that there is no normalization regarding hours worked, and the author states that "It could be due to the obvious reason that the rate of construction activity decreases on the weekends. The high rates (20%) on Tuesdays and Thursdays could not be explained."

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In summary, if we can use workplace accidents as indicators of overall concentration levels of workers and consequently manufacturing quality, the data suggest that there is overall little difference between weekdays, with Mondays and Fridays generally being best. How come? It may just turn out that most workers don't get totally wasted on the week-end, but actually relax, and that on Friday, the prospect of relaxing on the week-end makes people happier and thus more motivated to do a good job.

Jonas
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  • Is this data normalized to account for different numbers of work hours on different days of the week? I'd expect the fraction of accidents on a weekend to be less than 1/7, simply because fewer people work on weekends. Without such normalisation we can't conclude much about how the rate of accidents per worker-hour varies over the week. Also, is the "workers are more relaxed" explanation backed by anything, or is it pure speculation? – Nate Eldredge Sep 15 '15 at 01:08
  • @Nate: The data is not normalized. I have added a statement from the author. Also, the "workers more relaxed" explanation is speculation on my part; I assumed that "It may just turn out" would signal this appropriately. – Jonas Sep 15 '15 at 10:03