collecting and interpreting data Flashcards
what is a population?
a set of people or objects which is of interest for some question or experiment
what is a sample?
a subset of the population
what is a sampling frame?
a list of all the objects in a population available to be sampled
what is a sampling fraction?
the proportion of the population included in the sample
how do you calculate the sampling fraction?
size of sample / size of population
what is a census?
a sample consisting of the whole population
what does it mean if a sample is representative?
the proportion of objects in the sample with a defined characteristic is approximately equal to the proportion of objects in the population with this characteristic
what is simple random sampling?
takes a random selection from the population to use as a sample
how do you carry out a simple random sample?
suppose the population has size N, and the sample will be of size n
1. give each member of the population a different number from 1 to N
2. use a random number generator to select a number from 1 to N, and add the corresponding member of the population to the sample
3. repeat step 2 until there are n objects in the sample
4. ignore repeats
what is stratified sampling?
- the population is divided into strata based on a certain characteristic
- a sample of size n is fixed, the number of objects selected from each stratum is directly proportional to the size of the stratum
- a simple random sample, or systematic sample, is then taken from each stratum
- these samples are used collectively to form the whole population
what is systematic sampling?
takes the list and then takes every nth item from the list
how do you carry out a systematic sample?
population size = N, sample size = n
let K = N/n, rounded down to an integer
- give each member of the population a number from 1 to N
- select a number from 1 to N using a random number generator and start a list with this number
- add K to the most recent number in the listand add this number to the list. If you reach the end of the population, start again from the top
- repeat until there are n objects in the list
what is opportunity sampling?
when a sample is selected because the opportunity is there
what is cluster sampling?
- the area of interest is divided into subgroups
- a selection of these areas is made randomly, and all the members of the population in those areas form the sample
what is self-selected sampling?
occurs when people volunteer to take part
what is quota sampling?
- sample will contain a fixed number of people with a given characteristic
- the quota of items to be included from the different subgroups isn’t necessarily proportional
- opportunity sampling is used until the quota is filled
what is an advantage of simple random sampling?
every individual in the population has the same probability of being selected
what is a disadvantage of simple random sampling?
may not represent certain subgroups well if they are small in the population
what is an advantage of stratified sampling?
ensures representation from all subgroups in the population
what is a disadvantage of stratified sampling?
requires knowledge of the entire population to define strata properly
what is an advantage of quota sampling?
ensures representation from different subgroups + more flexible than stratified sampling
what is a disadvantage of quota sampling?
subjectivity in quota setting
what is an advantage of systematic sampling?
easy to implement and requires less time than simple random sampling
what is a disadvantage of systematic sampling?
if there is a pattern in the sample frame, the sample may not be representative of the entire population
what is an advantage of opportunity sampling?
cost-effective, often less expensive than the other sampling methods
what is a disadvantage of opportunity sampling?
potential for homogeneity, limits the diversity of the sample
what is an advantage of cluster sampling?
requires choosing selected clusters at random rather than evaluating entire populations, can save time and money
what is a disadvantage of cluster sampling?
increased sampling error due to the variability between clusters
what is an advantage of self-selected sampling?
quick and easy to obtain participants
what is a disadvantage of self-selected sampling?
results may not be generalisable to the larger population
