the easy one.
to select a random sample....
(1) assign a number to every member of the population.
(2) create a list of random numbers, e.g. by using a computer =, calculator or picking the numbers out of a bag.
(3) match the random numbers to members of the population.
the hard one.....
sometimes the population can be split into groups where the members have something in common, e.g. age groups or gender. in these casaes you can use STRATIFIED SAMPLING.
with this method, each groups share of the sample is calculated based on its share of the population-so bigger groups get more representation, and smaller groups get less.
to calculate the number of members from ea ch group
(1) find the proportion of the population contained in the group
number in group total population
(2) multiply it by the sample size