Define the population - start by defining the population and usually this is done by setting criteria and then estimating how many people or things that might be. For example, suppose I want to sample all second year students on computer courses who study mathematics and I estimate this to be 100 although in this case I might be able to get students records to tell me exactly how many there are but there are always drop outs so in a way it is an estimate.
Define a Sample Frame - this just means a list of some kind from which you will actually choose your sample points. In this case let us suppose I can get hold of class lists and it might look as follows where I number each student from 1 to 100 (in this case). In fact as long as you number this list systematically at this point you can start and finish anywhere - so you could number the frame from 200 to 299 or 87 to 186 and so on.
001 John Ashman
002 Paul Brigham
..
095 Janet Brown
...
100 Anthony Zaccari
Decide or Calculate a Sample Size - there are many ways to do this but just for example purposes let us say that I want to choose randomly 20 students for my research study out of the 100 I have available in my sample frame.
Generate Random Number - now I must generate 20 different random numbers between 1 and 100 (or between whatever systematic numbering for the frame you used). If the generator gives you the same number more than once just discard them. Here I use the Iphone app AppBox Pro (a tool box of apps) and use the one called Random by telling it my range (1 to 100) and then pressing a click icon (or you can shake the phone) and it give me the list:
68, 33, 61, 89, 17, 24, 73, 80, 01, 50, 85, 92, 60, 95, 37, 72, 79, 21, 28, 11
If you which and convenience write them out in order:
01, 11, 17, 21, 24, 28, 33, 37, 50, 60, 61, 68, 72, 73, 79, 89, 85, 89, 92, 95
DO NOT be tempted to tamper with this list so stupidly say to yourself things like 72 and 73 cannot be 'right'. You MUST trust that the Iphone app has done its job and given you a random list. I warn you, more problems that you can imagine occur when people try to second guess these sophisticated random number generators - just trust them.
Select the Sample - now go through you sample frame selecting the students based on these random numbers. Again BE WARNED do NOT try to second guess no matter how tempted you are. Thus we end up with our sample of 20 students.
001 John Ashman
011 Paul Aldridge
017 Victor Litchmore
021 Gaetan Madhvani
..
095 Janet Brown
These are now your selected
20 sample points. If you wish you can select a few more in case some refuse to take part
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