Epistemic spot checks typically consist of references from a book, selected by my interest level, checked against either the book’s source or my own research. This one is a little different that I’m focusing on a single paragraph in a single paper. Specifically as part of a larger review I read Ericsson, Krampe, and Tesch-Römer’s 1993 paper, The Role of Deliberate Practice in the Acquisition of Expert Performance (PDF), in an attempt to gain information about how long human beings can productivity do thought work over a time period.
This paper is important because if you ask people how much thought work can be done in a day, if they have an answer and a citation at all, it will be “4 hours a day” and “Cal Newport’s Deep Work“. The Ericsson paper is in turn Newport’s source. So to the extent people’s beliefs are based on anything, they’re based on this paper.
In fact I’m not even reviewing the whole paper, just this one relevant paragraph:
When individuals, especially children, start practicing in a given domain, the amount of practice is an hour or less per day (Bloom, 1985b). Similarly, laboratory studies of extended practice limit practice to about 1 hr for 3-5 days a week (e.g., Chase & Ericsson, 1982; Schneider & Shiffrin, 1977; Seibel, 1963). A number of training studies in real life have compared the efficiency of practice durations ranging from 1 -8 hr per day. These studies show essentially no benefit from durations exceeding 4 hr per day and reduced benefits from practice exceeding 2 hr (Welford, 1968; Woodworth & Schlosberg, 1954). Many studies of the acquisition of typing skill (Baddeley & Longman, 1978; Dvorak et al.. 1936) and other perceptual motor skills (Henshaw & Holman, 1930) indicate that the effective duration of deliberate practice may be closer to 1 hr per day. Pirolli and J. R. Anderson (1985) found no increased learning from doubling the number of training trials per session in their extended training study. The findings of these studies can be generalized to situations in which training is extended over long periods of time such as weeks, months, and years
Let’s go through each sentence in order. I’ve used each quote as a section header, with the citations underneath it in bold.
“When individuals, especially children, start practicing in a given domain, the amount of practice is an hour or less per day”
Generalizations about talent development, Bloom (1985)
“Typically the initial lessons were given in swimming and piano for about an hour each week, while the mathematics was taught about four hours each week…In addition some learning tasks (or homework) were assigned to be practiced and perfected before the next lesson.” (p513)
“…[D]uring the week the [piano] teacher expected the child to practice about an hour a day.” with descriptions of practice but no quantification given for swimming and math (p515).
The quote seems to me to be a simplification. “Expected an hour a day” is not the same as “did practice an hour or less per day.”
“…laboratory studies of extended practice limit practice to about 1 hr for 3-5 days a week”
Skill and working memory, Chase & Ericsson (1982)
This study focused strictly on memorizing digits, which I don’t consider to be that close to thought work.
This study had 8 people in it and was essentially an identification and reaction time trial.
Discrimination reaction time for a 1,023-alternative task, Seibel, R. (1963)
3 subjects. This was a reaction time test, not thought work. No mention of duration studying.
“These studies show essentially no benefit from durations exceeding 4 hr per day and reduced benefits from practice exceeding 2 hr”
Fundamentals of Skill, Welford (1968)
In a book with no page number given, I skipped this one.
Experimental Psychology, Woodworth & Schlosberg (1954)
This too is a book with no page number, but it was available online (thanks, archive.org) and I made an educated guess that the relevant chapter was “Economy in Learning and Performance”. Most of this chapter focused on recitation, which I don’t consider sufficiently relevant.
p800: “Almost any book on applied psychology will tell you that the hourly work output is higher in an eight-hour day than a ten-hour day.”(no source)
Offers this graph as demonstration that only monotonous work has diminishing returns.
p812: An interesting army study showing that students given telegraphy training for 4 hours/day (and spending 4 on other topics) learned as much as students studying 7 hours/day. This one seems genuinely relevant, although not enough to tell us where peak performance lies, just that four hours are better than seven. Additionally, the students weren’t loafing around for the excess three hours: they were learning other things. So this is about how long you can study a particular subject, not total learning capacity in a day.
Many studies of the acquisition of typing skill (Baddeley & Longman, 1978; Dvorak et al.. 1936) and other perceptual motor skills (Henshaw & Holman, 1930) indicate that the effective duration of deliberate practice may be closer to 1 hr per day
“Four groups of postmen were trained to type alpha-numeric code material using a conventional typewriter keyboard. Training was based on sessions lasting for one or two hours occurring once or twice per day. Learning was most efficient in the group given one session of one hour per day, and least efficient in the group trained for two 2-hour sessions. Retention was tested after one, three or nine months, and indicated a loss in speed of about 30%. Again the group trained for two daily sessions of two hours performed most poorly.It is suggested that where operationally feasible, keyboard training should be distributed over time rather than massed”
Typewriting behavior; psychology applied to teaching and learning typewriting, Dvorak et al (1936)
Inaccessible book.
The Role of Practice in Fact Retrieval, Pirolli & Anderson (1985)
“We found that fact retrieval speeds up as a power function of days of practice but that the number of daily repetitions beyond four produced little or no impact on reaction time”
Conclusion
Many of the studies were criminally small, and typically focused on singular, monotonous tasks like responding to patterns of light or memorizing digits. The precision of these studies is greatly exaggerated. There’s no reason to believe Ericsson, Krampe, and Tesch-Römer’s conclusion that the correct number of hours for deliberate practice is 3.5, much less the commonly repeated factoid that humans can do good work for 4 hours/day.
[This post supported by Patreon].
Aceso Under Glass 
What jumps out at me here is that the claim you started with conflated the following questions:
(1) What’s the returns curve on hours of work per day?
(2) What’s the returns curve on hours of practice per day?
These are not at all obviously the same question, and it’s a bad look to cite evidence for one as evidence for the other without explanation.
“Work” is so broad a term I don’t think it’s insane to reference 2 in regards to 1, although obviously people should be clearer what they’re answering.
The broadness of the term “work” is precisely why inferences about a narrow and unusual subset aren’t likely to be good descriptions of the whole.
It is about as silly as trying to figure out the distributions of heights in humans by measuring children.
I find it odd that the last sentence in the article is suddenly about work and not practice, learning. People are not robots, so there are other factors in play – passion, genuine interest in the topic, health (mental and physical), nutrition, environment, genetics, personal background, habits, etc. The problem is many societies as still stuck in the format created during the industrial revolution, where people were seen as nothing more than bio machines, operators, made to work for abnormal amount of hours per day with no rest (or very little).
I’ve not seen anyone enjoy an 8 hour desk job, or anyone who doesn’t have negative side effects from working such a job. Something like a 2-4 hours of highly focused work is probably going to be much more effective, than spreading that over a longer period. For any other kind of work that is mostly time consuming, but not challenging, automation is key.
As for practicing and learning, what I have found out in simracing is that the longer you rest between intensive sessions, the more your brain processes and the better the results. It becomes quite irrelevant and even counter-productive if you just drive for hours on end, every day, with little rest and little self-reflection, analysis, in between. Progress is never linear or constant, it’s more like we hit plateaus and to move onto the next one, requires time, rest, deep thought, attention to detail. Of course the factors I mentioned earlier can make a big difference.