statistics - sampling

how do you get a random sample?

• all members of the population have an equal chance of being drawn

• all samples have an equal chance of being selected

how do you extract simple random sample?

• number the list of the population (your sampling frame)

• generate 4 digit random numbers from a random number generator

• discard 0000 or any above the sampling frame and any repeats

• when you have your desired sample amount choose the members of the population that numbers correlate to the numbers chosen

how do you extract an unrestricted random sample?

• as a simple random sample process but the repeats are not discarded

how do you extract a proportional stratified sample?

• divide your required sample size in proportion to the sizes of the strata within your population using the formula : (number in strata/number in population) x sample size

• obtain a simple random sample from each strata

how do you extract a non proportional stratified sample?

• divide your required sample size in the proportions given in the question (it may be appropriate to over respresent one or more of the strata

• obtain a simple random sample from each strata

how do you extract a cluster sample?

• number the clusters and choose the required number of clusters using a simple random sample

• choose sample within each cluster via another simple random sample or use all members of the cluster dependent on required sample size

how do you extract a systematic sample?

• number the list of the population (your sampling frame)

• divide to population by sample size

• generate one random digit number between 000 and the interval calculated to use as a starting point

• your sample size is then selected by choosing the starting point followed by every number the interval goes up and lands on in the list after that

how do you extract a juddmental sample?

this is a non probability based method so it is not random and may include bias

• an expert in the fiels decides who would make good research subjects

how do you extract a snowball sample?

this is a non probability based method

• a member of the population who agrees to take part in the research invites other members of the population to take part in the research

• this may be the only method for accessinf members of a particular population