Data Collection Flashcards
Define population
The whole set of items that are of interest
Define 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
Define census
Every element of the population is surveyed which is rare, expensive and labour intensive
Define sampling unit
Individual units of a population (eg a resident)
Define sampling frame
List of sampling units formed when sampling units are numbered or named (eg list of residents)
Define random sampling method and identify the 3 types
Every element of the population has an equal chance of being selected. Stratified sampling, systematic sampling and simple random sampling
Explain a simple random sample
Need a sampling frame. Each person is allocated a unique number and a selection of these numbers is chosen at random (by generating random numbers using a computer or calculator or random number table, or by lottery sampling which involves numbers being written on tickets and the required number of tickets for the sample are drawn out)
Explain systematic sampling
The required elements are chosen at regular intervals from an ordered list but the first element is chosen at random. Eg if a sample size 20 was required from a population of 100, you would take every fifth person after the first person you picked (out of the first five) because 100/20 is 5
Explain stratified sampling
The population is divided into mutually exclusive strata (males and females for example) and a random sample taken from each , the proportion of each strata sampled should be the same. (Number in stratum divided by number in population) x overall sample size
Identify the 2 types of non random sampling
Quota and opportunity/convenience sampling
Explain quota sampling
Population divided into groups according to a givenm characteristic, size of each group determines the proportion of the sample that should have that characteristic. Interviewer meets people, discusses their group and allocates them into the correct quota, this continues until all quotas are filled (ignore people when the quota they fit into is full)
Explain convenience sampling
Taking a sample from people who are available at the time the study is carried out and who fit the criteria you are looking for
What are the 2 advantages of taking a census?
It should give a completely accurate result, everyone’s views will be known
What are the 4 disadvantages of a census?
Time consuming, expensive, cannot be used when the testing process destroys the item, hard to process large quantity of data
What are the 3 advantages of a sample?
Less time consuming and expensive than a census, fewer people have to respond, less data to process than a census
What are the 2 disadvantages of a sample?
Data may not be as accurate, sample may not be large enough to give information about small sub groups of the population
What are 3 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 4 disadvantages of simple random sampling?
Not suitable when the population size of the sample size is large, could be errors in sampling frame, person chosen from sampling frame could be absent on day of survey (if the survey is at work), gaining access to the whole sampling list can present challenges in schools/unis
What are the 3 advantages of systematic sampling?
Simple and quick to use, suitable for large samples and large populations, clustered selection eliminated (a phenomenon in which randomly chosen samples are uncommonly close together in a population, random samples can only deal with this by increasing the number of samples or running more than one survey which can be expensive alternatives)
What are the 3 disadvantages of systematic sampling?
The systematic method assumes the size of the population is available or can be reasonably approximated, population needs to exhibit a natural degree of randomness along the chosen metric because of the population has a type of standardized pattern the risk of accidentally choosing very common cases is more apparent, greater risk of data manipulation with systematic sampling because researchers might be able to construct their systems to increase the likelihood of achieving a targeted outcome rather than letting the random data produce a representative answer
What are the 2 advantages of stratified sampling?
Sample accurately reflects the population structure, guarantees proportional representation of groups within a population
What are the 4 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, overlapping can be an issue if there are subjects that fall into multiple subgroups, finding an exhaustive and definitive list of an entire population can be challenging
What are 4 advantages of quota sampling?
Allows a small sample to still be representative of the population (improves the representation of particular strata (groups) within the population, as well as ensuring that these strata are not over-represented), no sampling frame required so is quick and easy, inexpensive, allows for easy comparison between different groups within a population
What are 5 disadvantages of quota sampling?
Possible that the selection of units to be included in the sample will be based on ease of access and cost considerations resulting in sampling bias, not possible to make statistical inferences from the sample to the population, 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, each unit from the population must only belong to one stratum
What are the 2 advantages of opportunity sampling?
Easy way to obtain a large amount of data in a relatively short time frame, experiments with memory or situations in nature are examples where all members of the population may be highly similar so the method of sample selection is not of paramount importance
What are the 3 disadvantages of opportunity sampling?
Researcher is likely to pick people who make eye contact, smile, or give other nonverbal cues that they will likely consent to take the survey and to choose people similar to themselves both socially and culturally which can introduce a bias into the sample selection, using the convenience method may not be representative of the entire population from which it is drawn, no way when using an opportunity sample to assess how representative the sample is of the population being studied
What is the difference between continuous and discrete data?
A variable that can take any value in a given range is a continuous variable and they are usually obtained by measuring, a variable that can only take specific values in a given range is a discrete variable and these are usually obtained by counting
Where is Leuchars?
East coast of Scotland, longitude 02:86W, latitude 56:38N, altitude 10m
Where is Leeming?
Inland near the North of England, altitude 33m, longitude 01:53W, latitude 54:30N
Where is Heathrow?
Inland in South East England, altitude 33m, longitude 01:53W, latitude 54:30N
Where is Hurn?
South coast of England, altitude 10m, longitude 01:83W, latitude 50:78N
Where is Camborne?
South West coast of England, altitude 87m, latitude 50:22N, longitude 05:33W
Where is Perth and what is its weather like?
East coast of Australia, altitude 29m, latitude -31.933, 115.967. Almost always light wind speeds, a lot of dates with no rainfall but some with a lot
Where is Beijing and what is its weather like?
Inland in East China, altitude 55m, latitude 39.933, 116.283. Always light wind speeds, almost all dates with no rainfall
Where is Jacksonville and what is its weather like?
East coast of USA, altitude 9m, latitude 30.5, -81.7. Almost always light wind speeds, a lot of dates with no rainfall but some with a lot
What is daily mean temperature measured in and how?
Recorded by thermometers in a louvered screen 1.25 metres above short grass but at some weather centres and climate data logger stations observations are made from a non standard roof top enclosure, values noted in degrees and tenths, it is the average of the hourly readings 0900-0900GMT
What is daily total rainfall measured in and how?
Millimeters, a ‘tr’ is less than 0.05mm, refer to 24 hour periods commencing at 0900GMT on the date of entry, includes solid precipitation which has been melted to be measured as rainfall
What is daily total sunshine measured in and how?
Hours and tenths, shows the amount of bright sunshine on the day of entry, at most met office stations sunshine is measured by an instrument that mesdures the amount of solar radiation exceeding a threshold
What are daily mean wind speed, maximum gust speed, maximum gust direction and daily mean wind direction measured in and how?
Windspeed in knots (1 knot=1.15mph) and averaged over 24 hours from 0000GMT on date of entry, daily wind direction measured as an average over 24 hours and rounded to nearest 10 degrees, maximum gust speed is maximum instantaneous speed during 24 hours from 0000GMT, direction of maximum gust is direction from which wind was blowing when maximum gust occurred and is measured in degrees from true north, windspeed also categorised according to the Beaufort scale for all stations, for uk locations wind and gust direction measurements are also given as cardinal (NESW)
What is daily maximum relative humidity?
A measure of how close the air is to being saturated with water vapour, relative humidities of above 95% are associated with mist and fog
What is cloud cover?
The fraction of the celestial dome covered by cloud, measurred in eights (Oktas) so highest value can be 8, DISCRETE VARIABLE
What data is available for uk stations?
Daily mean temp, daily total rainfall, daily mean windspeed, daily mean total clouds, daily total sunshine, daily maximum gust, daily mean wind direction, daily maximum relative humidity, daily maximum gust direction, daily mean pressure
What is visibility and what is it measured in?
Measured horizontally, in decametres (Dm), it is the greatest distance at which an object can be seen in daylight and recognised, measured using vistometer at automatic sites but used to be done by observers at manual stations, at some weather centres and data logger stations observations are made from a non standard roof top exposure, unavailable data indicated by dash
What are 3 key things to remember?
The data is may to October so not representative of whole year round weather patterns, there are five uk stations and 3 overseas, large data set has gaps
What is pressure measured in and what is it?
Pascals (Pa). One hectopascal (hPa) = 1 millibar (mb). The mean sea level pressure which is calculated from a measurement made at station level
What data is available for non uk stations?
Daily mean air temp, daily mean pressure, daily mean wind speed, 24 hour rainfall total
