S&M - Data Collection Flashcards
what is a population?
a whole set of items that are in interest
what does a census observe or measure?
it observes or measures every member of a population
what is a sample
a selection of observations taken from a subset of the population who’s is used to find out information about the population as a whole
advantages of census
it should give a completely accurate result
disadvantages of census
-time consuming & expensive
-cannot be used when the testing process destroys the item
-hard to process large quantity of data
advantages of sample
-less time consuming & expensive than a census
-fewer people have to respond
-less data to process than in a census
disadvantages of 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
why can the size of the sample affect the validity of any conclusions drawn?
-the size of the sample depends on the required accuracy and available resources
-generally, the larger the sample, the more accurate it is, but you will need greater resources
-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 sampling units?
individual units of a population
what’s a sampling frame?
sampling units of a population individually named or numbered
a supermarket wants to test a delivery of avocados for ripeness by cutting them in half.
1) suggest why they shouldn’t cut them all in half
2) suggest one way they could improve their estimate
1) testing all would mean none left to sell, a census is not appropriate (when testing a product destroys it)
2) they could take a larger sample, this would give better estimate of overall proportion of ripe avocados (larger samples produce more accurate predictions about a population)
random sampling
-every member of population has equal chance of being selected
-sample therefore should be representative of the population
-it also helps to remove bias from a sample
what are the 3 methods of random sampling?
-simple random sampling
-systematic sampling
-stratified sampling
how to carry out simple random sampling?
-you need a sampling frame
-each person / thing is allocated a unique number and a selection of these numbers is chosen at random
-there are 2 methods of choosing numbers: generating random numbers & lottery sampling
-in lottery sampling, members of sampling frame could be written on tickets and placed into a hat
how to carry out systematic sampling
the required elements are chosen at regular intervals from an ordered list
eg if a sample size of 20 was required from a population of 100 you would take every fifth person since 100/20 = 5
