Data Sampling Flashcards
Population
Whole set of items that are of interest
Census
Observes and measures every member of the population
Sample
Selection of observations taken from a subset of the population which is used to find information about the population as a whole
Advantages and Disadvantages of using a Census
Advantages
- It should give a completely accurate result
Disadvantages
- Time consuming and expensive
- Cannot be used when testing process destroys the item
- Hard to process large quantity of data
Advantages and Disadvantages of using a Sample
Advantages
- Less time consuming and expensive
- Fewer people have to respond
- Less data to process
Disadvantages
- The data may not be as accurate
- The sample may not be large enough to give information about small sub-groups of the population
Sampling Unit
Individual units of a population
Sampling Frame
Sampling units of a population are individually named or numbered to form a list
Simple Random Sample
every sample of size n has an equal chance of being selected
Systematic Sampling
the required elements are chosen at regular intervals from an ordered list
Stratified Sampling
the population is divided into mutually exclusive strata and a random sample is taken from each
The number sampled in a stratum =
number in stratum/number in population x overall sample size
Advantages and Disadvantages of Simple Random Sampling
Advantages
- Free of bias
- Cheap and easy to implement for a small population and small samples
- Each sampling unit has a known and equal chance of selection
Disadvantages
- Not suitable when the population size or the sample size is large
- A sampling frame is needed
Advantages and Disadvantages of Systematic Sampling
Advantages
- Simple and quick to use
- Suitable for large samples and large populations
Disadvantages
- Sampling frame needed
- It can introduce bias if the sampling frame is not random
Advantages and Disadvantages of Stratified Sampling
Advantages
- Sample accurately reflects the population structure
- Guarantees proportional representation of groups within a population
Disadvantages
- Population must be clearly classified into distinct strata
Selection within each stratum suffers from:
- Not suitable when the population size or the sample size is large
- A sampling frame is needed
Quota Sampling
an interviewer/researcher selects a sample that reflects the characteristics of the whole population
Opportunity Sampling
consists of taking the sample from people who are available at the time of the study and fit the criteria you are looking for
Advantages and Disadvantages of Quota Sampling
Advantages
- Allows a small sample to still be representative of the population
- No sampling frame required
- Quick, easy and inexpensive
- Allows for easy comparison between different groups within a population
Disadvantages
- Non-random sampling can introduce bias
- Population must be divided into groups, which can be costly and inaccurate
- Non-responses are not recorded as such
Advantages and Disadvantages of Opportunity Sampling
Advantages
- Easy to carry out
- Inexpensive
Disadvantages
- Unlikely to provide a representative sample
- Highly dependent on individual researcher
Quantitative data
data associated with numerical observations
Qualitative data
data associated with non-numerical observations
Continuous variable
a variable that can take any value in a given range
Discrete variable
a variable that can take only specific values in a given range
If data is coded using the formula:
y = x-a/b
The mean of the coded data is given by:
ȳ = x̄-a/b
The mean of the coded data is given by ȳ= x̄-a/b
The original data is given by:
x̄ = bȳ+a
Standard deviation of the coded data
σy = σx/b
Taking a Simple Random Sample (4 marks)
- Identify the sampling frame
- Assign numbers to units 1 to n
- Use a random number generator and select sample numbers between 1 and n
- Use units corresponding to the numbers selected
