Sampling Sampling DesignDesign

Unit 5

Do frog fairy tale Do frog fairy tale p.89p.89

PopulationPopulation

the entire group of individuals the entire group of individuals that we want information aboutthat we want information about

CensusCensus

a complete count of a complete count of

the population (ALL)the population (ALL)

How good is a How good is a census?census?

frog fairy tale . . .frog fairy tale . . .

The answer is 83!The answer is 83!

Why would we not Why would we not use a census all the use a census all the

time?time?1)1) Not accurateNot accurate

2)2) Very expensiveVery expensive

3)3) Perhaps impossiblePerhaps impossible

4)4) If using destructive sampling, you would If using destructive sampling, you would destroy population destroy population

• Breaking strength of soda bottlesBreaking strength of soda bottles• Lifetime of flashlight batteriesLifetime of flashlight batteries• Safety ratings for carsSafety ratings for cars

Look at the U.S. census – it has a huge amount of

error in it; plus it takes a long to compile the data making the data obsolete

by the time we get it!

Suppose you wanted to know the average weight

of the white-tail deer population in Texas –

would it be feasible to do a census?

Since taking a census of any population takes

time, censuses are VERY costly to do!

SampleSample A part of the population that A part of the population that

we actually examine in order we actually examine in order to gather informationto gather information

Use sample to generalize to Use sample to generalize to populationpopulation

SampleSample

Sampling Sampling designdesign

refers to the refers to the methodmethod used to choose the used to choose the sample from the sample from the populationpopulation

Identify the sampling Identify the sampling designdesign

1)The Educational Testing Service 1)The Educational Testing Service (ETS) needed a sample of (ETS) needed a sample of colleges. ETS first divided all colleges. ETS first divided all colleges into groups of similar colleges into groups of similar types (small public, small types (small public, small private, etc.) Then they private, etc.) Then they randomly selected 3 colleges randomly selected 3 colleges from each group.from each group.

Stratified random Stratified random samplesample

Identify the sampling Identify the sampling designdesign

2) A county commissioner wants to 2) A county commissioner wants to survey people in her district to survey people in her district to determine their opinions on a determine their opinions on a particular law up for adoption. She particular law up for adoption. She decides to randomly select blocks decides to randomly select blocks in her district and then survey all in her district and then survey all who live on those blocks.who live on those blocks.

Cluster samplingCluster sampling

Identify the sampling Identify the sampling designdesign

3) A local restaurant manager wants 3) A local restaurant manager wants to survey customers about the to survey customers about the service they receive. Each night the service they receive. Each night the manager randomly chooses a manager randomly chooses a number between 1 & 10. He then number between 1 & 10. He then gives a survey to that customer, gives a survey to that customer, and to every 10and to every 10thth customer after customer after them, to fill it out before they leave.them, to fill it out before they leave.

Systematic random samplingSystematic random sampling

Sampling Sampling frameframe

a list of a list of everyevery individual in the individual in the populationpopulation

Multistage Multistage samplesample

select successively select successively smaller groups within smaller groups within the population in stagesthe population in stages

SRS used at each stageSRS used at each stage

To use a multistage approach to sampling RHS students, we could first divide 2nd period classes by

level (AP, Honors, Regular, etc.) and randomly select 4 second period

classes from each group. Then we could randomly select 5 students from each of those classes. The

selection process is done in stages!

BiasBias A systematic error in A systematic error in

measuringmeasuring the estimate the estimate

favors certain outcomesfavors certain outcomesAnything that causes the data to be wrong!

It might be attributed to the researchers, the

respondent, or to the sampling method!

Sources of Sources of BiasBias

things that things that can causecan cause bias in your samplebias in your sample

cannot do anything cannot do anything with bad datawith bad data

Voluntary Voluntary responseresponse

People chose to People chose to respond respond

Usually only people Usually only people with very strong with very strong opinions respondopinions respond

An example would be the surveys in magazines that ask readers to mail in the survey.

Other examples are call-in shows, American Idol, etc.

Remember, the respondent selects themselves to

participate in the survey!

Remember – the way to determine

voluntary response is:

Self-selection!!

Convenience Convenience samplingsampling

Ask people who Ask people who are easy to askare easy to ask

Produces bias Produces bias resultsresults

An example would be stopping friendly-looking people in the

mall to survey. Another example is the surveys left on

tables at restaurants - a convenient method!

The data obtained by a convenience sample will be

biased – however this method is often used for surveys &

results reported in newspapers and magazines!

UndercoveragUndercoveragee

some groups of some groups of population are left population are left out of the out of the sampling processsampling process

Suppose you take a sample by

randomly selecting names from the phone

book – some groups will not

have the opportunity of being selected!

People with unlisted phone numbers – usually high-income families

People without phone numbers –usually low-income families

People with ONLY cell phones – usually young adults

NonresponseNonresponse occurs when an individual occurs when an individual

chosen for the sample chosen for the sample cancan’’t be contacted or t be contacted or refuses to cooperaterefuses to cooperate

telephone surveys 70% telephone surveys 70% nonresponsenonresponse

People are chosen by the researchers, BUT refuse to

participate.

NOTNOT self-selected!

This is often confused with voluntary response!

Because of huge telemarketing efforts in the past few years,

telephone surveys have a MAJOR problem with

nonresponse!

One way to help with the problem of nonresponse is to make follow contact with

the people who are not home when you first contact

them.

Response biasResponse bias occurs when the occurs when the behavior of respondent behavior of respondent or interviewer causes or interviewer causes bias in the samplebias in the sample

wrong answerswrong answers

Suppose we wanted to survey high school students on drug

abuse and we used a uniformed police officer to

interview each student in our sample – would we get honest

answers?

Response bias occurs when for some reason (interviewer’s or

respondent’s fault) you get incorrect answers.

Wording of the Wording of the QuestionsQuestions

wording can influence wording can influence the answers that are the answers that are givengiven

connotation of wordsconnotation of words use of use of ““bigbig”” words or words or technical wordstechnical words

Questions must be worded as neutral as possible to

avoid influencing the response.

The level of vocabulary should be appropriate for

the population you are surveying

–.

– if surveying doctors, then use more

complex, technical wording.

– if surveying Podunk, TX, then you should avoid complex vocabulary.

Source of Bias?Source of Bias?1) Before the presidential election of 1) Before the presidential election of 1936, FDR against Republican ALF 1936, FDR against Republican ALF Landon, the magazine Landon, the magazine Literary DigestLiterary Digest predicting Landon winning the election predicting Landon winning the election in a 3-to-2 victory. A survey of 2.8 in a 3-to-2 victory. A survey of 2.8 million people. George Gallup million people. George Gallup surveyed only 50,000 people and surveyed only 50,000 people and predicted that Roosevelt would win. predicted that Roosevelt would win. The DigestThe Digest’’s survey came from s survey came from magazine subscribers, car owners, magazine subscribers, car owners, telephone directories, etc.telephone directories, etc.

Undercoverage – since the Digest’s survey comes from car owners, etc., the people selected were mostly from high-income families and thus mostly Republican! (other answers are possible)

2) Suppose that you want 2) Suppose that you want to estimate the total to estimate the total amount of money spent by amount of money spent by students on textbooks each students on textbooks each semester at SMU. You semester at SMU. You collect register receipts for collect register receipts for students as they leave the students as they leave the bookstore during lunch one bookstore during lunch one day.day.

Convenience sampling – easy way to collect data

orUndercoverage – students who

buy books from on-line bookstores are included.

3) To find the average 3) To find the average value of a home in value of a home in Plano, one averages the Plano, one averages the price of homes that are price of homes that are listed for sale with a listed for sale with a realtor.realtor.

Undercoverage – leaves out homes that are not for sale or

homes that are listed with different realtors.

(other answers are possible)