Census
observes and measures every member of a population
Sample
a selection of observations taken from a subset of the population which is used to find out information about the whole population
Advantages and Disadvantages of a Census
+ should give a completely accurate result
- time consuming, expensive, hard to process large amount of data and cannot be used when testing process destroys item
Advantages and Disadvantages of a Sample
+ less time consuming, more cost effective than census, fewer people have to respond, less data than census
- data is not as accurate, sample may not be large enough to give info about small sub-groups of a population
Simple Random Sampling
a sample size of n where every sample size of n has an equal chance of being selected
Systematic Sampling
required elements are chosen at regular intervals from an ordered list
Stratified Sampling
population is divided into mutually exclusive strata and a random sample is taken from each
Advantages and Disadvantages of Simple Random Sampling
+ free of bias, easy and cheap to implement for small populations and small samples, each sampling unit has a known and equal chance of selection
- not suitable when the population size or the sample size is large, a sampling frame is needed
Generating a sampling frame ; 2 Methods
generating random numbers and lottery sampling
Advantages and Disadvantages of Systematic Sampling
+ simple and quick to use, suitable for large samples and large populations
- sampling frame is needed, can introduce bias if the sampling frame is not random
Advantages and Disadvantages of Stratified Sampling
+ sample accurately reflects the population structure, guaruntees proportional representation of groups within a population
- population must be clearly classified into distinct strata, again may not accurately represent small subsets within a large population
Quota Sampling
interviewer or researcher selects a sample that reflects the characteristics of the whole population
Opportunity Sampling
taking the sample from people who are available at the time the study is carried out and who fit the criteria you are looking for
Advantages and Disadvantages of Quota Sampling
+ allows a small sample to be representative of population, no sampling frame required, quick, easy and inexpensive, different groups within population are easily comparable
- non random sampling can introduce bias, pop. divided into groups which can be costly or inaccurate, increasing scope of study increases number of groups, which increases time and expense, non-responses are not recorded as such
Advantages and Disadvantages of Opportunity Sampling
+ easy to carry out, inexpensive
- unlikely to provide a representative sample, highly dependant on individual researcher
Quantitative Variables/Data
variables/data associated with numerical observations
Qualitative Variables/Data
variables/data associated with non-numerical observations
Continuous Variable
variable that can take any value in a given range
Discrete Variable
variable that can only take specific values in a given range
