brm class presentation

8/7/2019 BRM class presentation

1/19


2/19

Simple Random Sampling: A samplingprocedure that assures each element in thepopulation an equal chance of being included

in the sample. E.g. drawing names from a hat,winning raffle ticket from a large drum etc.

If sample is large computer based randomsampling maybe used for sample selection.


3/19

e.g. A researcher may be interested in selecting asimple random sample of all presidents of savingsand loan associations in New Mexico.

For this each presidents name is assigned anumber from 1 to 95 and each no is written on apiece of paper.

All slips are placed in a drum and thoroughlymixed.

One is selected for each sampling unit.

If sample size is 45, the selection procedure isrepeated 44 times after the first slip has beenselected.


4/19

Def: A sampling procedure in which an initialstarting point is selected by a random process,and then every nth number on the list is

selected. E.g. selecting every 23rd name from a rural

telephone directory that does not separatebusiness listings from household listings


5/19

Random results if the arrangement of the itemsin the list is random in character.

Periodicity: Occurs if list is not random incharacter.

E.g. collecting retail sales every seventh daywould result in a distorted sample because

there be a systematic pattern of selectingsampling units, sales for only one day of theweek would be sampled


6/19

Def: A probability sampling procedure inwhich simple random subsamples are drawnfrom within different strata that are more or

less equal on some characteristic.Advantages:

More efficient sample

Random sampling error reduced Assurance that sample will accurately reflect

the population on the basis of the criterion


7/19

Step 1: Identify a variable as an efficient basisfor stratification.

Characteristics of variable chosen:

a) characteristic of the population elementsknown to be related to dependent variable.

b) increase homogeneity within each stratum

c) increase heterogeneity between strata.d) easily convertible into sub groups.

E.g. pharmaceutical company


8/19

Step 2: For each separate sub group, a list ofpopulation elements must be obtained.

Step 3: A separate simple random sample istaken within each stratum by using a table ofrandom numbers or some other device.


9/19

Def: A stratified sampling in which the numberof sampling units drawn from each stratum isin proportion to the population size of that

stratum. A proportionate sample would have the same

percentages as in the population.


10/19

Def: A stratified sample in which the samplesize for each stratum is allocated according toanalytical considerations.

The general logic is that as variability increases,sample size must increase to provide accurateestimates.


11/19

Def: An economically efficient sampling technique inwhich the primary sampling unit is not the individualelement in the population but a large cluster ofelements.

Area Sample: Def: A cluster sample in which the primary sampling

unit is a geographic area. Most popular type of cluster sampling. E.g. a grocery researcher may randomly view all, or a

sample of, grocery stores within the geographicclusters. Interviews are confined to these clusters, no interviews

occur in other clusters.


12/19

Cluster sample are utilized when no lists ofsample population are available.

Cluster should be homogenous as the

population itself.


13/19

Def: Sampling that involves using acombination of other probability samplingtechniques.

E.g. A political pollster investigating an electionin Arizona may follow the following steps:

Step 1: Choose counties within the state

Step 2: Precincts within the selected counties

maybe chosen. Step 3: Blocks within the precincts might be

chosen, then all the blocks within thegeographic area would be interviewed.


14/19

Degree of accuracy

Resources

Time

Advance knowledge of population

National versus Local project

Need for statistical analysis


15/19

Rapid reach to a large sample

Instantaneous

Day-of-the-week effect

Lack of Internet penetration

A select sample segment


16/19


17/19

A sample from a panel

High response rate

Incentivized

Assurance

Propensity-weighting scheme


18/19

Recruited ad hocSamples

Opt-in Lists


19/19

brm class presentation

Documents