The number of elements in a sample is called as sample size.
A survey based on the responses of a sample of individuals, rather than the entire population.
In a cluster sample, the entire population is divided into heterogeneous group and some of such groups are selected as sample which is chosen on geographical basis is example of cluster sampling. If the blocks are chosen separately from different strata, so the overall design is a stratified cluster sample.
A sample drawn because of its convenience not a probability sample e.g. sample of people having telephone numbers in a city to decide about the population in city is convenience sample. It is selected because it would be easier to interview people over phone rather than visiting their homes. Samples of convenience are not representative of the population, and it is not possible to quantify how unrepresentative results based on samples of convenience will be.
A random sample is a sample whose members are chosen at random from a given population in such a way that the chance of obtaining any particular sample can be computed..
Simple Random Sample or Probability Sample.
A simple random sample is the sample selected from population where every individual of the population has equal chance of getting selected. A simple random sample can be drawn in two ways – SRSWR (Simple Random Sample With Replacement) and SRSWOR (Simple Random Sample With Out Replacement), In SRSWR individual once selected in the sample can be again selected in another sample i.e. it is put back in the population. In SRSWOR individual once selected in the sample cannot be selected in any other sample i.e. it is not put back in the population. If we want to draw sample of size 2 from the numbers 4.5.6 then the with SRSWR we can have the following samples. (4,4), (4,5), (4,6), (5,4), (5,5), (5,6), (6,4), (6,5), (6,6). While with SRSWOR we can have only 3 samples as (4,5) (4,6), (5,6) Thus if sample size in n i.e if n units are to be drawn from a population of N units, then the total number of
samples that can be drawn by SRSWR method is Nn While with SRSWOR method we can draw NCn samples.