Data Collection And Sampling Methods Flashcards
What is a population?
The whole set of items that are of interest.
What does a census measure?
Every member of a population.
What is a sample?
A selection of observations taken from a subset of the population which is used to find out information about the population as a whole.
What are the advantages of a census?
It should give a completely accurate result.
What are the disadvantages of a census?
-time consuming and expensive.
-cannot be used when the testing process destroys the item
-hard to process large quantities of data
What are the advantages of a sample?
-quicker and cheaper than a census
-fewer people need to respond
-less data to process than a census.
What are the disadvantages of a sample?
-the data may not be as accurate
-the sample may not be large enough to give information about small subgroups of the population.
What is are sampling units?
Individual units of a population.
What is a sampling frame?
Sampling units are numbered to form a list.
What is a parameter?
A number that describes the entire population.
What is a statistic?
A number taken from a single sample.
How does the size of a sample effect the validity of conclusions?
•Generally the larger the sample, the more accurate it is
•If the population is very varied you need a larger sample than if the population were uniform.
•Different samples can lead to different conclusions due to the natural variation in a population.
What are the 6 types of sampling?
•simple random sampling
•systematic sampling
•stratified sampling
•quota sampling
•opportunity / convenience sampling
•cluster sampling (a sample from a sample)
What is simple random sampling?
Every sample has an equal change of being selected.
Method of simple random sampling?
-in a sampling frame each item is given an identifying number
-use a random number generator (on your calculator) to generate different random numbers.
-match numbers with corresponding sampling units.
What are the advantages of simple random sampling?
•bias free
•easy and cheap implement
•each number has a known equal chance of being selected.
What are the disadvantages of simple random sampling?
•not suitable when population size is large.
•sampling frame needed.
What is systematic sampling?
Required elements are chosen at regular intervals in ordered list.
Method of a systematic sampling?
•each item in given an identifying number
•find k by doing - population size ÷ sample size
• start at a random number between 1 and k take every kth element.
What are the advantages of systematic sampling?
•simple and quick to use
•suitable for large samples/populations
What are the disadvantages of systematic sampling?
•sampling frame needed
•can introduce bias if sampling frame not random
•not random as not every item has an equal chance of being selected.
What is satisfied sampling?
Population divided into groups (strata) and a simple random sample carried out in each group.
What is a strata?
Groups that a population is divided into.
Method of stratified sampling?
•allocate each sampling units in each group with a number.
•sample the same proportion from each strata. Using - (no. in strata ÷ no. in population) × sample size
• use a simple random sample to select the units from each strata.
What are the advantages of stratified sampling?
•reflects population structure
•guarantees proportional representation of groups within a population
•has random selection within an individual group to reduce bias.
What are the disadvantages of stratified sampling?
•population must be clearly classified into distinct stata.
•selection within each stratum suffers from same disadvantages as simple random sampling.
•(ie. sampling frame needed, not suitable for large samples)
What are the 3 types of random sampling?
• simple random sampling
• systematic sampling
• stratified sampling
What are the 3 types of non-random sampling?
• quota sampling
• opportunity / convenience sampling
• cluster sampling
What is quota sampling?
Population divided into groups according to characteristic.
Method of quota sampling.
A quota of items / people in each group is set to try and reflect the groups proportion in the whole population.
Interviewer selects the actual sampling units.
proportion isn’t always necessary
What are the advantages of quota sampling?
-Allows small sample to still be representative of population.
-no sampling frame required
-quick,easy,inexpensive.
-allows for easy comparison between different groups in population.
What are the disadvantages of quota sampling?
-non random sampling can introduce bias.
-population must be divided into groups, which can be costly or inaccurate.
-increasing scope of study increases number of groups, adding time / expenses.
-non responses are not recorded.
What is Opportunity / convenience sampling?
Sample take from people who are available at time of study, who meet criteria.
Method of opportunity / convenience sampling.
Eg: take the first 20 people you meet outside a shop on a Monday morning who are carrying shopping bags.
What are the advantages of opportunity / convenience sampling?
-easy to carry out
-inexpensive
What are the disadvantages of opportunity / convenience sampling?
-unlikely to provide a representative sample.
-highly dependent on individual researcher
What is cluster sampling?
Split the population into relevant groups. Choose a sample from these groups. From these samples chosen take a sample of the required size.
What are the advantages of cluster sampling?
-easy to carry out
-inexpensive
What are the disadvantages of cluster sampling?
-May not be representative of the population.
-non random sampling can introduce bias.
