Where c depends on how confident you want to be that your margins will include the population value.Sampling
Sample
Population or the Universe
A summary measure such as the
meanin the sample is called: statistic,
in the population or universe it is called: parameter.
Why sampling?
Cheaper
Possible
Better quality control over measurement
Destructive sampling
Sampling Methods
Non-random Samples
-- Accidental or convenience samples
-- judgement or purposive sample
(we decide what is typical)
-- quota samples
Random Samples
Each member of the population has an equal (or known) chance of being in the sample
simple random sample
needs full list
-- table of random numbers
-- systemic samples
-- stratified samples
-- cluster samples
primary, secondary etc. sampling units
Sampling Error
The error that is due to the fact that the sample is selected by a chance process
For percentages the sampling error (or the margin of error) is
Where c depends on how confident you want to be that your margins will include the population value.
If you are happy with a 95% confidence then c = 1.96
If you want a 99% confidence then c =2.58