5.3: SAMPLING

Errors in SamplingSampling Errors- Errors caused by the act of taking a sample. Makes sample results inaccurate.

Random Sampling ErrorErrors caused by the chance in selecting a random sample.

Nonsampling ErrorErrors not related to the act of selecting a sample from the population. They can even be present in a census.

Sampling Frame:Individuals from the population from which the

sample can be obtained from.Not always the whole population.EX: A political group interested in how people in

the state feel about budget cuts takes a sample of 1000 people from registered voter lists.

SAMPLING ERRORS:

Undercoverage

• When some groups of the population are left out of the sample

• On purpose or by accident

• Examples: • Call-in poll, stopping people @ mall, mailings,

standing by back door of school, etc.

Processing Errors* Mistakes* Examples:

- doing arithmetic wrong- typos- recording wrong numbers/info- losing data

NONSAMPLING ERRORS:

Response Error

* When subjects give an incorrect response

* Examples:- lying (especially with sensitive questions)- remembering info incorrectly- don’t understand question- etc.

Nonresponse Error* Failure to obtain data from an individual in the sample

* Happens b/c subjects refuse to respond or can’t be contacted

* Examples:- not answering phone/ hanging up phone- not sending back mailing- absent on day of poll- refuse to write answers on a survey

Wording of questions

* When a question:

- is confusing

- uses big words or technical language that most people don’t understand

- uses a word that has more than one meaning and doesn’t clarify

- ARE SLANTED towards one response (based on the question alone or a statement with the question

* Usually non-sampling error

What to do about these errors:* For nonresponse… just select another individual

* Weight the responses based on who responds

Example: If twice as many rural homes respond than urban homes, give more weight to the responses from the urban.

Complete the book problems with a partner:p. 242 #56p. 246 #64p. 252 #75

p. 242 #56

56) (a) non-sampling(b) non-sampling(c) sampling (undercoverage, not everyone gets

the paper)

64) 50% in favor was for the question worded “protecting the life of the unborn child”

29% in favor was for the question “prohibiting abortions”

p. 246 #64

p. 252 #75

75) (a) sampling error (undercoverage on those not in the phone book)

(b) non sampling error (non response)

(c) sampling error (convenience sample, voluntary response sample)

Other types of samples…

Review:

Take an SRS of 5 people from the following list. Number the people down the columns, starting with 1. Start at line 124 in the Table of Random Digits.

John Christine RobertSue Matthew StacyLauren Andrea MeganJim Chris PatrickMarie Gretchen AlisonJake Jennifer JeffreyKaren Frank Annie

Other ways to take samples

Example: At a university, there are 1500 professors. However, 1000 are male and 500 are female. To take an accurate sample of the university faculty’s opinions, we decide to take a sample of 100 male professors and 50 female. Their total sample is these 150 professors.

Why is this not an SRS? What was their process? Why is this sampling method better than just an SRS?

Stratified Random Sample: (NOT simple random sample)

1) Divide population into groups with something in common (called STRATA)

Example: gender, age, etc.

2) Take separate SRS in each strata and combine these to make the full sample

- can sometimes be a % of each strata

Try p. 246 #62

Basketball:WadeCarterMillerStoudamireGasolFarmarBillups

Golf:ElsSinghKim

Probability Sample:

* Chosen by chance* Give a chance to each individual, but not always

an equal chance.* We must know what chance each sample has

Example: The lottery

Systematic Sample:

* Randomly picking a first individual, and then selecting every ____ person after that.

* Example: Interview every 5th person that walks thru the back door of CB South in the morning.

* Example: Write down the type of car in every 3rd spot in the CB South parking lot.

Cluster Sample

• Pick a certain area/part of the population.• Try to get every individual in that selection

Census

• Sample everyone in the entire population.

Questions to ask before you believe a poll:

p. 249

Try the following:

p. 246

#65, 68

Try the following:

p. 253 – 256

#77--81, 83, 86

77) (a) Biased because of undercoverage. You are not able to sample the most expensive seats

(b) Sampling error, because it was an error in the WAY you collected your sample

78) # 19, 26, 06,09Rodriguez, Montoya, Fernandez, Castillo

79) (a) The population that they want info about is the opinions of all residents of West Lafayette, Indiana

(b) Definitely larger. Since it was a call-in poll, there was voluntary response, and undercoverage.

80) (a) the population is all American college students(b) the list of 340 PSY 001 students

(c) If he is trying to conclude about ALL college students, you should not just sample from students in PSY 001.

(d) Yes, it is slanted. “fair price to pay”

(e) He did not ask students about whether they were in favor of TV commercials. He asked them if they thought commercials were a fair price to pay for watching TV.

(f) No, not relevant. The sampling frame does not represent the population.

81) (a) – give each student a number- randomly select 250 students to interview

(b) - Randomly choose a number from 1 thru 10. - once you have that number (like 7 for example), take

every 10th person from that (like 17th, 27th, 37th, etc.)

(c) Separate into bussed and live nearby. Give every student in the bussed category a #, and then randomly select 200. Then

give every student in the live nearby category a number, and randomly select 50. You total of 250 is your sample.

(d) stratified. It is a more accurate representation of the population.