The identification and certification of a Blue Zones™ area or group is based on demographic criteria that are country-specific and depending on available documentation and its reliability.
First criterion
The reliability of individual longevity should be thoroughly proved and therefore the extreme ages of the oldest olds, dead or still alive should be proved based on ad hoc documentation. Without documentation like birth and death records and continuous population registration system, that reliability cannot be ensured.
Second criterion
The longevity of the population at a national level should be among the highest ranked at world level based on demographic indicators that are life expectancies (computed both by period and cohort) and population age and sex structure as observed in censuses.
Third criterion
If the two previous criteria are met, a “Blue Zones” area might be certified within such country as the place defined spatially as a sum of local administrative entities where the population experience a higher longevity compared to the rest of the country.
The indicators to be used are various and depend on data availability. The potential list of indicators include the following ones:
- Life expectation at birth (both by period or cohort, having in mind that due to the continuous improvement of longevity, only the same year of observation can be compared strictly)
- Life expectation at age 50
- Life expectation at age 80
- Probability to survive from 50 to 80
- Probability to survive from 80 to 90
- Probability to reach 90 from birth
- Probability to reach 100 from birth
- The proportion of nonagenarians or centenarians alive in the total population might not be used as indicators except with extreme caution, as important biases may exist as demonstrated in scientific papers.
- Each indicator should be considered separately for males and females and the sex ratio that is also considered as an indicator to certify a BLUE ZONES branded region or group.
Each indicator should be compared with its value at national level as disseminated by the National Institution for Statistics or the Human Mortality Database. Appropriate statistical tools should be used to ensured that the various indicators show value that are significantly higher considering the size of the population and the absolute difference with the National standard.
A spatial smoothing aggregation method might be used to a Blue Zones hotspot as a spatial contiguous area.
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