Populations And Samples Flashcards
Population
Whole set of items that are of interest.
Census
Observes or measure every member of a population.
Sample
Selection of observations taken from a subset of the population which is used to find out information about the population as a whole.
Advantages of a census
It should give a completely accurate result
Disadvantages of a census
Time consuming and expensive
Cannot be used when the testing process destroys the item
Hard to process large quantity of data
Advantages of a sample
Less time consuming and expensive than a census
Fewer people have to respond
Less data to process than in a census
Disadvantages of a sample
The data may not be as accurate
The sample may not be large enough to give information about small sub-groups of the population
How to carry out simple random sampling
Each object is assigned an number. A random number generator is used, and the numbers produced are assigned to objects which are then picked.
How to carry out systematic sampling
Required elements are chosen at regular intervals in ordered list.
Take kth elements where k = pop size/sample size starting at random item between 1 and k.
How to carry out stratified sampling
Divide the population into different strata, and a simple random sample is Carried out in each group.
Samp size/pop size (sampled from each strata)
Advantages of random sampling
Bias free
Easy and cheap to implement
Each member has an equal chance of selection
Disadvantages of random sampling
Not suitable for large population
Sample frame needed
Can introduce bias if sampling frame is not random
Advantages of systematic sampling
Simple and easy to use
Suitable for large samples
Disadvantages of systematic sampling
Sampling frame needed
Can introduce bias if sampling frame is not random
Advantages of stratified sampling
Reflects population structure
Guarantees proportional representation within population
