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I recently heard a claim that a new case of dementia is diagnosed every 4 seconds from various resources including

The total number of new cases of dementia each year worldwide is nearly 7.7 million, implying 1 new case every 4 seconds.

Worldwide someone is diagnosed with dementia every 4 seconds (Alzheimer's Disease International).

This is difficult for me to believe. That would mean there are (60 * 60 * 24 / 4 =) 21,600 new cases of dementia diagnosed every day. Is there any basis for this claim?

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    Let's do a ballpark calculation. There are over 7 billion people on Earth, most of whom live for less than 100 years, so we could guesstimate than about 1% of them die every year. (This is an underestimate, since the world population is growing -- the real figure is closer to 0.85% -- but it's good enough for a rough calculation.) Thus, we get 70 million deaths per year, or a bit under 200,000 deaths per day (the real figure is closer to 150,000), or about 2 per second (real figure ≈ 1.7). If just one in 8 people get diagnosed with dementia before they die, that's consistent with your figure. – Ilmari Karonen May 20 '15 at 8:09
  • It seems from the answer below that the first quote is correct. So the "diagnosed" part of the second quote is probably wrong, since a huge percentage of the world's population doesn't have enough access to healthcare to ever be diagnosed with dementia [citation needed]. – Oscar Cunningham May 20 '15 at 12:38
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The source you cite from the WHO is fairly authoritative, so the best I can do is point to other sources that are consistent with that source.

"Approximately 469,000 people age 65 or older will develop Alzheimer’s disease in the United States in 2014." (https://www.alz.org/downloads/Facts_Figures_2014.pdf at p.19)

That is approximately 1300 / day.

The United States population is approximately 1/20 of the world population – naively multiplying gives an estimated world-wide incidence Alzheimer's of approximately 26000 / day.

Thus the estimate of a diagnosis every 4 seconds is consistent with the incidence of Alzheimer's disease in the United States.

The World Alzheimer Report from 2009 (http://www.alz.co.uk/research/world-report-2009) estimated a slightly lower incidence:

With a new case of dementia in the world every seven seconds there is no time to lose.

[...]

with 4.6 million new cases annually

That number is supported by (Ferri, Cleusa P., et al. "Global prevalence of dementia: a Delphi consensus study." The Lancet 366.9503 (2006): 2112-2117.):

Applying the DISMOD estimated incidence rates, we estimate 4.6 million new cases of dementia every year (about one new case every 7 s).

  • +1 for the peer-reviewed Lancet estimate (do you have a link?). I would lead with that, and let your original calculations be a back-up. – Oddthinking May 20 '15 at 2:11

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