Stats T1 Flashcards
What happens in random sampling?
Every member of the population has an equal chance of being selected. The sample should therefore be representative of the population
What does random sampling remove?
It removes bias from the sample
What are the 3 methods of random sampling?
- Simple random sampling
- Systematic sampling
- Stratified sampling
What happens in simple random sampling?
A simple random sample size of n is one where every sample size n has an equal chance of being selected
What do you need to carry out a simple random sample?
You need a sampling frame, usually a list of people or things
Each person allocated a unique number & selection of these numbers is chosen at random
What are the two methods of choosing numbers in simple random sampling?
- Generating random numbers
- Lottery sampling
What is lottery sampling?
The members of the sampling frame could be written on tickets & placed into a ‘hat’
The required no. tickets would then be drawn out
What happens in systematic sampling?
The required elements are chosen at regular intervals from an ordered list
If a sample size od 20 was required from a population, how would you take the correct sample size?
You would take every fifth person
How must you choose the first person in systematic sampling?
They must be chosen at random
What happens in stratified sampling?
The population is divided into mutually exclusive strata (ie males and females) & a random sample is taken from each
What should the proportions be like from strata in stratified sampling?
The proportion of each strata should be the same
What is the formula used to work out the amount of people we should sample from each stratum?
No. in stratum
————————– x overall sample size
No. in population
What are the advantages of simple random sampling?
- Free of bias
- Easy & cheap to implement for small populations & small samples
- Each sampling unit has a known & equal chance of selection
What are the disadvantages of simple random sampling?
- Not suitable when the population size or the sample size is large
- A sampling frame is needed
What are the advantages of systematic sampling?
- Simple and quick to use
- Suitable for large samples & large populations
What are the disadvantages of systematic sampling?
- Sampling frame is needed
- It can introduce bias if the sampling frame is not random
What are the advantages of stratified sampling?
- Sample accurately reflects the population structure
- Guarantees proportional representation of groups within a population
What are the disadvantages of stratified sampling?
- Population must be clearly classified into distinct strata
- Selection within each stratum suffers from the same disadvantages as simple random sampling
What are the two types of non-random sampling?
- Quota sampling
- Opportunity sampling
What is quota sampling?
An interviewer/ researcher selects a sample that reflects the characteristics of the whole population
How is the population divided in quota sampling?
Divided into groups according to a given characteristic
What does the size of each group with a certain characteristic in quota sampling determine?
The size of each group determines the proportion of the sample that should have that characteristic
What is the interviewer’s role in quota sampling?
They would meet people, assess their group and then, after interview, allocate them into the appropriate quota
What happens if a person refuses to be interviewed in quota sampling?
(or if the quota they fit into is full) You simply ignore them and move on to the next person
What is opportunity sampling?
Consists of taking the sample from people who are available at the time the study is carried out & who fit into the criteria you’re looking for
Give an example of people who would be used in opportunity sampling
The first 20 people outside a supermarket on a Monday morning who are carrying shopping bags
What are the advantages of quota sampling?
- Allows small sample to still be representative of the population
- No sampling frame required
- Quick, easy & cheap
- Allows for easy comparison between different groups in a population
What are the disadvantages of quota sampling?
- Non-random can introduce bias
- Population must be divided into groups - can be costly/inaccurate
- Increasing scope of study increases number of groups, which adds time and expense
- Non-responses are not recorded
What are the advantages of opportunity sampling?
- Easy
- Cheap
What are the disadvantages of opportunity sampling?
- Unlikely to provide a representative sample
- Highly dependant on individual researcher
What are the two types of data?
Quantitative
Qualitative
What is quantitative data?
Variable or data associated with numerical observations (aka quantitative variables)
What is qualitative data?
Variables or data associated with non-numerical observations (aka qualitative variables)
Give an example of a quantitative variable
Numbered shoe size
Give an example of a qualitative variable
Hair colour (you can’t give a number to it)
What is a continuous variable?
A variable that can take any value in a given range
Give an example of a continuous variable
Time can take any value - eg 2 seconds, 2.1 seconds, 2.01 seconds etc
What is a discrete variable?
A variable that can take only one specific value in a given range
Give an example of a discrete variable
The number of girls in a family - because you can’t have 2.56 girls
How can large amounts of data be displayed?
In a frequency table or as grouped data
How is data presented in a grouped frequency table?
When data is presented in a grouped frequency table, the specific data values are not shown
What are the groups in grouped frequency tables more commonly called?
Classes
What do class boundaries in grouped frequency tables tell you?
They tell you the max and min values that belong in each class
What is the midpoint of class boundaries in grouped frequency tables?
The average of that class boundary
What is the class width in grouped frequency tables?
The difference between the upper and lower class boundaries
