Stats - sampling

population

every member of a group

parameter

number used to describe the population

A census

Survey that collects information from the entire population

Sample

subset of the population

Statistic

A number used to describe a sample

sample survey

collects information from a sample

sampling units

individual units for a population

sampling frame

A list of individually named or numbered units

Census advantages+ disadvantages.

A - accurate view of population
D - can’t do for infinite population, expensive, time consuming, cannot be used when testing involves destruction

Sample survey A and D

A - Quick,cheap, can be used for infinite pop.
D - not as accurate, data not large enough to represent small sub-groups, misleading info on whole pop.

sample mean

a statistic that is an estimate for the parameter

Random Sampling methods

Simple random, systematic, stratified

Non. random sampling

Quota,Cluster,opportunity

What is. simple random sampling

method where every sample has an equal chance of being selected

Method for simple random

Allocate identifying number to each unit in sampling frame.
Use random number generator to pick them.

A and D simple random sampling

A - bias free, easy + cheap to implement, each. number has a known equal chance of being selected.
D - nnot suitable when pop. size is large, sampling frame is needed

What is systematic sampling

Required elements are chosen at regular intervals in ordered lists

Method for systematic sampling

Take every kth element where
k = pop.size/sample size. starting at a random time between 1 and k

A and D systematic sampling

A - simple, quick to use, suitable for large samples + populations
D - sampling frame needed, can introduce bias if sampling frame is not random

What is stratified sampling

pop. is divided into strata and a simple random sample is carried out in each group. Is used when a sample is large and population is naturally divided into groups

Method for stratified sampling

sample size/pop.size and is sampled from each strata

A and D stratified sampling

A - reflects pop.structure, guarantees proportional representation of groups with pop.
D - pop. must be clearly classified into distinct strata, same. disadvantages as simple random sampling

Method for quota sampling

Divide pop. into groups according to characteristics
A quota of units in each group is set to try and reflect the group’s proportion in the whole pop.
Interviewer selects the actual sampling units

Advantages for Quota sampling

representative of population
no sampling frame
quick easy inexpensive
easy comparison between groups

Disadvantages for quota sampling

can introduce bias

non-responses not recorded

pop. must be divided into groups = greater cost and inaccuracy

Method for opportunity sampling

sample taken from units that are available at time of study(who meet the criteria)

advantages of opportunity sampling

no sampling frame needed
quick easy inexpensive

disadvantages of opportunity sampling

unlikely to produce a representative sample
bias is introduced as researcher chooses where and when to go.

What are clusters

natural subgroups within a pop.

Method for cluster sampling

random sample of clusters carried on the whole pop. and each are then sampled