data collection

what is a population?

the whole set of items that are of interest

what is a census?

a survey observing or measuring 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 sampling units?

individual units of a population

what is a sampling frame?

sampling units of a population that are individually named and numbered

what are the 2 main types of sampling?

• random sampling

• non-random sampling

what is random sampling?

• each member of the population has an equal chance of being selected

• simple random, systematic, stratified

how many types of random sampling are there?

3

what are the types of random sampling?

• simple random

• systematic

• stratified

what are the positives of random sampling?

• theoretically representative of the population

• removes bias from a sample

why is random sampling theoretically representative of the population?

because every member of the population has an equal chance of being selected

why is it good that random samples help to remove bias from a sample?

the researcher’s bias in choosing a sample could affect the validity of the sample, as it may not be representative of the population

negatives of random sampling?

what is a simple random sample?

every sample has an equal chance of being selected

how do you carry out a simple random sample?

• using a sampling frame, each person / thing is allocated a unique number and the selection of said number is random

• numbers can be picked using a random number generator, or via lottery sampling (putting names in a hat)

how is each participant numbered in a simple random sample?

by putting them in a sampling frame, then allocating unique numbers

how can participants be picked out in a simple random sample after being numbered?

• random number generator

• lottery sample

what is a lottery sample?

putting names in a hat and drawing out enough for the required sample size

what is used to carry out a simple random sample?

a sampling frame

why is a sampling frame used in a simple random sample?

in order to allocate a unique number to each person / thing in the sampling frame in order to randomly select numbers and therefore participants

how many names should you pick out in a lottery sample?

the required sample size

why is it an advantages that each sampling unit in a simple random sample has an equal chance of selection?

because, theoretically, the sample should be representative of the population

what are the advantages of simple random sampling?

• free of bias

• easy and cheap to implement for small populations and small samples

• each sampling unit has a known and equal chance of selection

what are the disadvantages of simple random sampling?

• not suitable when the population size or the sample size is large as it’s potentially time consuming, disruptive, and expensive

• sampling frame needed

what is a systematic sample?

• the required elements are chosen at regular intervals from an ordered list

how is a systematic sample chosen?

• at regular intervals, found from population / sample sample

• the first person chosen is random

how should the first person in a systematic sample be chosen?

randomly

why should the first person in a systematic sample be chosen randomly?

come back to this

how do you decide the interval to use in a systematic sample?

population / sample size = interval

• e.g., if a sample of 20 was required from a population of 100, you take every 5th person since 100 / 20 = 5

what characteristic should the sampling frame have in a systematic sample?

it should be random to avoid bias

why should the sampling frame be random in a systematic sample?

to avoid bias

what are the advantages of systematic sampling?

• simple and quick to use

• suitable for large samples and large populations

what are the disadvantages of systematic sampling?

• sampling frame needed

• can introduce bias if the sampling frame is not random

what is a stratified sample?

the population is divided proportionally into mutually exclusive strata, and random samples are taken from each

what should be the same in a stratified sample?

the proportion of each strata sampled

how is the population divided in stratified sampling?

• into mutually exclusive strata

• according to their proportion in the population

how do you calculate the number of people that should be sampled from each stratum in stratified sampling?

number sampled in a stratum = (number in stratum) / (number in population) x overall sample size

what are the stratum in stratified sampling?

mutually exclusive strata - e.g. males and female

in stratified sampling, how do you pick a sample from the mutually exclusive stratums?

randomly

what is strata / stratum?

the groups in which the population is divided into in stratified sampling

what are the advantages of stratified sampling?

• sample accurately reflects 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

how should you answer a question asking you to describe the advantages / disadvantages of using a particular sampling type?

in context to the question

which sampling types require a sampling frame?

• simple random

• systematic

why should bias be avoided?

can reduce the validity? ask

what is non-random sampling?

sampling that isn’t random.. fuck man.. you’re so stupid

what is quota sampling?

an interviewer / researcher selects a sample that reflects the characteristics of the whole population

how is a quota sample chosen?

• interviewer meets people, assess their group, and allocate them into the appropriate quota

• population divided into groups based on certain characteristics

• size of group determines proportion of the same that should have that characteristic

how is the population found in quota sampling?

via an interviewer, and until the quota is filled

how are groups divided in quota sampling?

by desired characteristic

what determines the proportion of a certain characteristic within a quota sample?

the size of the group of the desired characteristic in relation to the entire population

what are the advantages of quota sampling?

• 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

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, which adds time and expense

• non-response are not recorded as such

why do non-random sampling methods introduce more bias than random?

smth abt the interviewer being a picky bitch

in quota sampling, when would a potential participant be ignored?

• when the person doesn’t want to be interviewed

• when the quota is full

in quota sampling, what happens when the quota is full?

• any potential participants of the desired characteristic is ignored

• a sample based on the proportion of the group is ready to be taken (providing all quotas are full)

stratified vs quota

what is opportunity sampling?

the sample is selected from people who are available at the time and fit the desired criteria (e.g., a street interview)

what is opportunity sampling also known as?

convenience sampling

what are the advantages of opportunity sampling?

• easy to carry out

• inexpensive

what are the disadvantages of opportunity sampling?

• unlikely to provide a representative sample

• highly dependent on individual researcher

what is quantitative data?

data regarding numbers

what is qualitive data?

data regarding words

what is continuous data?

data that can take any value in a given range

what is discrete data?

data that can take specific values in a given range

what is a grouped frequency table?

here

when is a grouped frequency table used?

when you have large amount of continuous data in classes

what are classes?

groups of continuous data that are given as a range, where specific data values are not known. used in group frequency tables

what are the class boundaries?

the maximum and minimum values that belong in each class

HERE

when do we rewrite data from a frequency diagram into it’s class boundaries?

• when the data is continuous

• when it has intervals between it’s classes

HERE

what are the class boundaries for continuous data in a grouped frequency table?

minimum value ≤ x < maximum value

what is the midpoint in a grouped frequency table?

the average of the class boundaries

what is the class width?

the difference between the upper and lower class boundary

what do you have to ensure before working with a grouped frequency table?

that, if the data is continuous, it is presented with class boundaries

here

what are the advantages of a census?

it should give a completely accurate result

what are the disadvantages of a census?

• time consuming

• expensive

• can’t be used when the testing process destroys the item (e.g., how many ropes break under strain)

• hard to process large quantities of data

what are the 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

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 sub-groups of the population

what does the validity of the conclusions depend on?

the sample size

what does the sample size depend on?

the required accuracy and available resources

what is an advantage of a larger sample?

generally more accurate results

what is a disadvantage of a larger sample?

more resources are needed, therefore greater cost

when do you generally need a larger sample?

when the population has lots of variety

when do you generally don’t need a larger sample?

when the population is uniform

which generally requires a larger sample - a varied or uniform population?

varied population

which generally requires a smaller sample - a varied or uniform population?

uniform population

why can different samples lead to different conclusions?

because of the natural variance in a population

what are the recorded data?

• daily mean temperature

• daily total rainfall

• daily total sunshine

• daily mean wind direction and windspeed

• daily maximum gust

• daily maximum relative humidity

• daily mean cloud cover

• daily mean visibility

• daily mean pressure

what are the recorded data for overseas locations?

• daily mean temperature

• daily total rainfall

• daily mean pressure

• daily mean windspeed

in the large data set, where is the weather data provided from?

• 5 uk weather stations

  1. Leuchars

  2. Leeming

  3. Heathrow

  4. Hurn

  5. Camborne

• 4 overseas weather stations

  1. jacksonville (usa)

  2. beijing (china)

  3. perth (australia)

what are the 5 uk weather stations in the large data set?

1. leuchars

2. leeming

3. heathrow

4. hurn

5. camborne

here

what are the 3 overseas weather stations in the large data set?

1. jacksonville (usa)

2. beijing (china)

3. perth (australia)

here

what is the furthermost north weather station in the uk?

leuchars

what is the furthermost west weather station in the uk?

camborne

what is the furthermost east weather station in the uk?

heathrow

what is the furthermost south weather station in the uk?

camborne

what is the furthermost west overseas weather station?

jacksonville (usa)

what is the furthermost east weather station?

beijing (china)

what is the furthermost south overseas weather station?

perth (australia)

what is the furthermost north overseas weather station?

beijing (china)