Selecting a sample manually (raffle tickets)
When choosing a random sample, each population member must have the same chance of being included in the sample. How can we select a random sample in practice? One method of selecting a random sample of size n is...
This method is often used for raffles, but thorough mixing is difficult for large populations and it is rarely used in research applications.
Random digits
An alternative method of selecting a random sample involves generating random digits (0, 1, ..., 9). There are several ways to generate random digits such that each has the same chance of appearing.
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Concatenating 2 or more of these random digits gives a larger random number.
Click the button Generate digit to generate a random digit.
Concatenating three such random digits gives a random number between 0 and 999. Click Generate value to find a random number in this range.
Random number between 0 and k
A random number that is equally likely to be any number between 0 and 357 can be found by repeatedly generating 3-digit numbers (between 0 and 999) until a value between 0 and 357 is obtained.
It is easier however to use a spreadsheet such as Excel — it has a function designed for this purpose, "=RANDBEWEEN(0, 357)".
Selecting a random sample
To select a random sample without replacement using random numbers,
The diagram below illustrates sampling without replacement from a population of 56 individuals. They have been numbered from 0 to 55.
Click Random index. If the resulting two digits are between 0 and 55, the corresponding individual is added to the sample. Otherwise, an error message appears and a new random value must be generated.
Repeat several times to add more individuals to the sample. Note that: