UNIT 3 PART A

Question 1

How can you estimate the probability of an event occurring?

Answer 1

run a simulation. Find the percent of trials that you observed the event occur.

Question 2

How many trials should you run to have an accurate simulation?

Answer 2

At least 20-30.

Question 3

Is it always better to do a census or a sample?

Answer 3

It depends. generally, it is better to do a sample since a census is expensive to execute, and because popultaions are always changing it is hardly more accurate then a sample. But, For small populations, a census is fine.

Question 4

To make a survey to tell of a restaurant is good, would you ask the people coming out of the restaurant?

Answer 4

People at the restaurant are probably there because they already like it. If you asked the question “Is this your first time dining here?” and if they say “yes” you survey them, that would be a better method.

Question 5

What are humans bad at ?

Answer 5

Humans are bad at generating random numbers.

Question 6

What are the 3 ways we used random numbers?

Answer 6

1. To simulate the likelihood of an event occurring. (ch 11)
2. To choose a sample that is representative of the population and avoid bias.(Ch 12)
3. To assign subjects (experimental units) to treatments to evenly distribute variability and help reduce possible confounding variables.(Ch 13)

Question 7

what is a simulation?

Answer 7

MIMICS REALITY.. In this class, using random numbers to show how random real world events may occur.

Question 8

when does a trial of a simulation end?

Answer 8

Generally there are two cases:

1. You want to know the probability of having x successes in n attempts (getting 3 smokers in a group of 5 students). Trials end when you get to n (get to 5 students). You record the number of smokers for each trial.
2. You want to know how many attempts it takes to get f successes. Trials end when you get f successes. Record the number of attempts.

Question 9

You want to simulate the likelihood of more than 4 psychology majors being on a full bus that seats 30. 1 in 9 students are psych majors.

Answer 9

use single digits on a random number table. Each digit represents a student on the bus. Ignore the zeros. Let 1 be a psych major, and 2 through 9 be other students. Trials end when you have reached 30 students. Count the number of psych majors (ones) in the trial. Record this. Do this 20 times. Find the percent of times there were 4 or more psych majors on the “bus.” If this occured in 5 trials.. then the likelihood is 5 in 20, or 25%

Question 10

What is the “response variable” in a simulation?

Answer 10

If you are wondering “how many batteries do you have to check to to find 3 bad batteries?” the response variable is “average number of batteries you checked” in the trial. If you are wondering “how many failed septic systems can be expected on a street of 10 houses?” the response variable is “the average number of failed systems” in the trial.

Question 11

How do you use a table of random digits for a simulation?

Answer 11

FIRST.. Make a key showing what the digits represent, whether you will use single, double or triple digits, and which, if any will be ignored (like repeats, or say “unique”)
SECOND.. Decide when a trial will end (after 12 events, or after 12 successes)
THIRD. Make sure to clearly label the successes and where the trials end on the table.
FOURTH: make sure you clearly say what you will be counting (number of tries to get n successes, or number of successes in n tries)

Question 12

When would you use two digits instead of a single on a random number table?

Answer 12

When the percent is not a multiple of ten. Like “18% of dogs eat underwear”. You’ll have to assign 01-18, or 00-17 as undie eating dogs.

Question 13

When can you use single digits for simulations?

Answer 13

When the percent is a multiple of ten. like “30% of teachers secretly twerk”, then you would assign 1-3 or 0-2 as twerking teachers.

Question 14

What is random sampling?

Answer 14

When we use chance to select a sample. We use a a randomizer. In random sampling, each subject has equal chance of getting selected.

Question 15

How can you simulate a coin flip with random number table?

Answer 15

Assign heads to odd numbers and tails to even numbers.

Question 16

How can you simulate rolling 1 die with a random number table

Answer 16

use only the digits 1-6, ignore 0, 7, 8, 9

Question 17

How can you simulate on your calculator?

Answer 17

RANDINT( lowest, highest, how many you want to grab)

Question 18

What are some Sampling Method ?

Answer 18

SRS, stratified, clustered, systematic, multistage, convenience, voluntary

Question 19

What are the two types of observatinal studies?

Answer 19

Retrospective, and Prospective

Question 20

What is a mutlistage sample?

Answer 20

A sample that combines several sampling methods

Question 21

In which sampling methods do the subjects have equal chances of being selected?

Answer 21

SRS, Stratified, Clustered, Systematic, and multistage.

Question 22

What is a quality of SRS that is not a quality of Systematic, Stratified or Clustering?

Answer 22

In an SRS, all groups are possible, and ALL POSSIBLE GROUPS have the same chance of being picked (like all senior male students.).The other methods have lots of “impossible groups” SRS has no impossible groups.-Stratified- an impossible group would be all girls (you’re taking some boys and girls)-Clustered- an impossible group would be all girls (each cluster has boys and girls)-systematic- an impossible group would be 4 people that are right next to eachothe (you are taking er on listvery nth person)

Question 23

What is a simple random sample?

Answer 23

ALL THE SUBJECTS IN A HAT!!! (or all names or numbers, if not, you’d need a large hat). A sample where every possible group has the same chance of becoming a part of a sample. You could get “all males”

Question 24

What is difference between subject and experimental unit?

Answer 24

Humans who are experimented on are commonly called subjects in an experiment. Subjects like dogs, days, plants and anything not human are called Experimental Units

Question 25

What is prospective study?

Answer 25

Prosepctive study is when you study the experimental unit’s present and futrue response variable.

Question 26

What is response bias? How do you avoid it?

Answer 26

Response bias is any influence that may sway the respondent to give a more favorable answer e.g wording of the question, interviewer’s behavior/background. Therefore, in a survey, ask questions that allow respondents to answer comfortably and honestly. Keep the wording “indifferent” or neutral in some way in order to unduly favor one response over another.

Question 27

What is retrospective study?

Answer 27

A retrospective study is a study that looks backwards in time. They focus on estimating differences between groups or variable association because they are not based on random samples.

Question 28

What is sampling error?

Answer 28

IT IS NOT A MISTAKE!!! Because the data in samples are generally different, the statistics calculated from one sample to another vary and are generally not equal to the parameter. This variablilty of the STATISTICS is called sampling error. (not the variability of the data).

Question 29

What is sample size and how does it compare with the fraction of a population?

Answer 29

Sample size is the number of individuals in a sample. The sample size determines how well the sample represents the population, not a fraction of the population sampled. The fraction of the population that you’ve sampled doesnt matter. Its the sample size its self thats most important.

Question 30

Does a sample of 128 people tell you more about a population of 1000 people or a million people?

Answer 30

It will tell you just as much about both. Same reliability (if sample is representative)

Question 31

What is statistically significant?

Answer 31

When an observed difference is too odd for us to believe that it is likely to have occurred naturally (or just randomly). Basically it is Statistically Significant when we don’t think it happened randomly. when you think “something’s up” or “something’s fishy”

Question 32

What is systematic sampling?

Answer 32

EVERY Nth. TOTAL POP/SAMPLE SIZE= your n. Randomly choose first. RANDINT(1, n). And then take every NTH.

Question 33

What is difference between non response bias and undercoverage?

Answer 33

You may ask someone to take a survey, they may say no. They may feel differently than the people who decide to take the survey. In this case, that is non-response bias. Undercoverage happens when you didn’t even ask some people to take the survey. The people you didn’t even ask might feel different.

Question 34

What is a sampling frame? (how is it diff from population?)

Answer 34

It is the frame from which you get your sample.Suppose you are wondering how elderly people on the cape feel about a new medicare law. If you go to nursing homes and randomly sample residents, then the frame is “elderly people at those nursing homes.” Your population is still elderly people on cape cod.

Question 35

What is the difference between response bias and nonresponse bias?

Answer 35

Response is when the person’s response is influenced by the question or questioning method (like if a parent asks if you use drugs, as opposed to a friend… there is only one answer to this, but one might respond differently to them), non response is is when the people who don’t respond might have different opinions/views than the people who did.

Question 36

What is the problem with convenience sampling?

Answer 36

The sample may not be representative as it is not randomized to include every type of person. Friends and family are convenient but they likely share similar opinions and thus the sample is not representative of a population.

Question 37

What is the standard sampling method?

Answer 37

A Simple Random Sample (SRS) is our standard. Every possible group of n individuals has an equal chance of being our sample. That’s what makes it simple. Put the names in a hat.

Question 38

What is undercoverage?

Answer 38

Undercoverage is when a group of the population is not represented in the sample.

Question 39

What is wrong with using volunteers in a survey?

Answer 39

Those who volunteer may not be like the rest of the population. An example may be, if you’re trying to find our how often people volunteer for things. So you ask for volunteers to take the survey. A question may be “when was the last time you volunteered for something?” Well. they all just volunteered for the survey!

Question 40

What’s the difference between a prospective and a retrospective study?

Answer 40

A retrospective study takes a group and looks back at its history while a prospective study watches a group for a period of time and records the data. RETRO-REVERSE, PROspective- PResent and On..

Question 41

What’s the difference between stratified and cluster sampling?

Answer 41

Stratified- you divide the population up into groups according to traits, called strata (groups with similar traits- homogeneous groups) and randomly choose a few from each strata.
Cluster- grab clusters of the population. each cluster should be like the population. use the whole cluster (usually)

Question 42

what’s the difference between response bias and nonresponse bias?

Answer 42

response bias is anything in a survey design that influences responses falls under the heading of response bias (wording of questions). Nonresponse bias is bias introduced to a sample when a large fraction of those sampled fails to respond.those who respond are likely to not represent the entire sample.

Question 43

Why do you have to Stratify?

Answer 43

You don’t have to.. But you might want to if you feel that a simple random sample might not be representative of the population . You want your sample to be like the population. a representative sample (it represents the population well).

Question 44

How can the WORDING of the question lead to response bias

Answer 44

Words or phrases that impact your feelings tend to influence responses. Look for “devastating, horrific, wonderful? etc.” Sometimes there is a background story like “Many americans lose jobs to illegal aliens every year, do you feel this is fair”

Question 45

Can a simulation be stimulating?

Answer 45

yes

Question 46

Give an simulation example where you would ignore some numbers and repeaters. (If you had to choose 3 people to go on a trip, What is the likelihood of choosing 3 girls randomly from a group of 8 people where half are girls)

Answer 46

Simulate likelihood of randomly choosing 3 girls from a group of 8 students where half are girls and half are boys. Let 0-3 rep girls. 4-7 rep boys. Ignore 8&9. Trials end when you get three UNIQUE integers that are not 8 or 9(ignore repeaters). So, if the table had 0 1 8 1 5 it would be G G X X B. You would ignore the 8 (not a person) and the second 1 (already chose her). Trials end when you get 3. Count how many trials had GGG and divide by total number of trials.