How can the primacy index of a countrys population be calculated?

The primacy index can be calculated by the following formula: PI = C1/ (C1 + C2 + C3 + C4) * 100 PI = primacy index C1 = amount of people in the biggest city of the country C2 = amount of people in the second city of the country C3 = amount of people on the third city of the country C4 = amount of people in the fourth city of the country So you divide the population of the biggest city of the country by the sum of the four biggest cities in the country and multiply that by 100. If the PI is > or = to 50, then it is a primate city.