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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?

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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, 2011 at 21:05
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    Where is the "born on date" on a car?
    – Hendy
    Jun 17, 2011 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, 2011 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, 2011 at 0:01
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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, 2011 at 8:29

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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.

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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, 2015 at 1:08
  • @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, 2015 at 10:03

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