note of 10/25/2018

more mearsurement of population
Net Migration Rate: the difference between the number of people entering and leaving a country during a year.
an excess of people entering the country is immigration.
written as positive number
EX: Canada has 5.65 migrants per 1000 population.
       United states: 2.45（it was 3.62 in 2015）
an excess of persons leaving the country is net emigration.
written as a negative number.
example: Mexico has -1.68 migrants per 1000 population
                                   - 3.61 two years ago, -3.11 last year
why do people migrate
push forces
civil war
environmental degradation
unemployment or underemployment
religious or ethnic persecution
pull force
better economic opportunity
better health services
religious freedom
political freedom