This measure may be explained by pointing out that it follows the same principle as the concept of median age.

The median age of a population indicates the central or middle value of age when all the persons in the population are listed from the lowest to the highest or from the highest to the lowest, so that half of the population falls below and half the population above it.

Thus dividing the entire population into two halves with respect to age. Similarly, it is possible to find out the size of a locality where the median inhabitant lives.

This index may be obtained with the help of the following formula: is the cumulative per cent of the population for the locality size just under 50 per cent; is the cumulative per cent of the next locality size category is the upper limit of the locality size i; and is the upper limit of locality size.

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This measure may be interpreted as meaning that the bigger locality sizes of the median inhabitant, the greater the degree urbanisation. It is, however, important to keep in mind that this index should be calculated only when the urban population in a country is more than 50 per cent of the total population.