Transcript
Page 1: Get out homework. Get out notes

•Get out homework.

•Get out notes.

Page 2: Get out homework. Get out notes

SECTION 5 .1 CONTINUED

Designing Samples

Page 3: Get out homework. Get out notes

Sample Designs

A simple random sample (SRS) of size n contains n individuals from the population chosen so that every set of n individuals has an equal chance of being selected.

Page 4: Get out homework. Get out notes

Sample Designs

Example: SRS or not? I want a sample of nine students from the

class, so I put each of your names in a hat and draw out nine of them. Does each individual have an equal

chance of being chosen?

Does each group of nine people have an equal chance of being chosen?

Page 5: Get out homework. Get out notes

Sample Designs

Example: SRS or not? I want a sample of nine students from the

class but I know that there are three juniors and 17 seniors in class, so I pick one junior at random and eight seniors. Does each individual have an equal chance

of being chosen?

Does each group of nine people have an equal chance of being chosen?

Page 6: Get out homework. Get out notes

Sample Designs

Better than a hat: Software can choose an SRS from a

list of the individuals in a list.Not quite as easy as software, but still

better than a hat: a table of random digits.

Computers.

Page 7: Get out homework. Get out notes

Sample Designs

A table of random digits is a long string of the digits 0 – 9 with two properties: Each entry in the table is equally likely to be

any of the ten digits 0 through 9 The entries are independent of each other.

(Knowing one part of the table tells you nothing about the rest of the table.)

TABLE B in the back of your book.

Page 8: Get out homework. Get out notes

Sample Designs

Each entry is equally likely to be 0 – 9

Each pair of entries is equally likely to be 00 – 99

Each triple of entries is equally likely to be 000 – 999

And so on…

Page 9: Get out homework. Get out notes

Sample Designs

Read example 5.4 on p. 276

Page 10: Get out homework. Get out notes

Sample Designs

A stratified random sample first divides a population into groups of similar individuals called strata. Then separate SRS’s are chosen from each group (stratum) and combined to make the full sample.

Page 11: Get out homework. Get out notes

Cautions about samples

Choosing samples randomly eliminates human bias from the choice of sample, but… What problems may remain?

Brainstorm

Page 12: Get out homework. Get out notes

Cautions about samples

Undercoverage Having an inaccurate list of the population

Who is excluded from a survey of “households”?

Who is excluded from a telephone survey?

Page 13: Get out homework. Get out notes

Cautions about samples

NonresponseOccurs when selected individuals

cannot be contacted or refuse to cooperate.

Page 14: Get out homework. Get out notes

Examples

Which problem (undercoverage or nonresponse) is represented? It is impossible to keep a perfectly

complete list of addresses for the U.S. Census: Homeless people do not have addresses In 1990, 35% of people who were mailed

Census forms did not return them.

Page 15: Get out homework. Get out notes

Response Bias

Results may be influenced by behavior of either the interviewer or the respondent.

Page 16: Get out homework. Get out notes

Response Bias

How might response bias show up in these situations? A survey about drug use or other illegal

behavior Questions asking people to recall events,

like:“Have you visited the dentist in the last six months?”

Page 17: Get out homework. Get out notes

Response Bias

The wording of questions can often lead to bias: “It is estimated that disposable diapers

account for less than 2% of the trash in today’s landfills. IN contrast, beverage containers, third-class mail, and yard wastes are estimated to account for 21% of the trash in landfills. Given this, in your opinion, would it be fair to ban disposable diapers?”

Page 18: Get out homework. Get out notes

Response Bias

“Does it seem possible or does it seem impossible to you that the Nazi extermination of the Jews never happened?”

22% said possible“Does it seem possible to you that the Nazi

extermination of the Jews never happened or do you feel certain that it happened?

1% said possible

Page 19: Get out homework. Get out notes

Inference about the Population

Even if we can eliminate most of the bias in a sample, the results from the sample are rarely exactly the same as for the population. Each different sample pulls different

individuals, so results will vary from sample to sample

Results are rarely correct for the population

Page 20: Get out homework. Get out notes

Inference about the Population

Since we use random sampling, we can use the laws of probability (later chapters!) We’ll be able to figure out the margin of

error (also in later chapters!)

Page 21: Get out homework. Get out notes

Inference about the Population

Just know now: larger random samples give more accurate results than smaller samples.

Page 22: Get out homework. Get out notes

Homework

p. 274 & 279 # 7 – 12

You do not need your book on Friday.


Top Related