UNIT 3 SAMPLING AND EXPERIMENTS AND STUFF Flashcards
How can you estimate the probability of an event occurring?
run a simulation. Find the percent of trials that you observed the event occur.
How many trials should you run to have an accurate simulation?
At least 20-30.
Is it always better to do a census or a sample?
It depends. generally, it is better to do a sample since a census is expensive to execute, and because populataions are always changing it is hardly more accurate then a sample. BUT?. For small populations, a census is fine.
To make a survey to tell of a restaurant is good, would you ask the people coming out of the restaurant?
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. But then again.. the people wouldn’t go into an Italian restaurant if they didn’t like that type of food.
What are humans bad at ?
Humans are bad at generating random numbers.
How can you “randomly ask 5 people” in the hall a question?
You could roll dice, and if it says 5.. Then the fifth person to walk by you interview, then roll again.. Repeating this is random, but remember that the sampling frame is just the people who walk down that particular hallway?
Sampling frame.
The group who you may sample from. If you do a phone survey then your sampling frame is only people with phones. If you are interested in everyone, a phone survey could suffer from undercoverage?
What are the 3 ways we used random numbers?
- 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)
what is a simulation?
Basically a test based on reality with a sequence of random outcomes that model it. Like an imitation.
when does a trial of a simulation end?
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.
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.
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%
How do you use a table of random digits?
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? 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.
When would you use two digits instead of a single on a random number table?
When the percent is not a multiple of ten.. Like “18% ofdogs eat underwear”.. You’ll have to assign 01-18, or 00-17 as undie eating dogs. All other digit pairs will be non-underwear eating dogs.
When can you use single digits for simulations?
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. Or to simulate rolling dice (1-6 faces, ignore 0, 7, 8, 9), or flipping coin (odds H, evens T)
Use the following words in one run on sentence: inference, sample, statistic, parameter, population, census, data
I was curious about a population parameter, but a census was too costly, so I collected data from a sample, calculated a statistic and used that to make an inference about the parameter of interest.
What is random sampling?
When we use chance to select a sample, when you use an actual randomizing mechanism.. Not your “random” guess! When you “randomly” do something in your head, it is not random. roll dice, shuffle cards, pick from a hat or use a random number table or calculator
How can you simulate a coin flip with random number table?
Assign heads to odd numbers and tails to even numbers.
How can you simulate rolling 1 die with a random number table
use only the digits 1-6, ignore 0, 7, 8, 9
How can you simulate on your calculator
RANDINT( lowest, highest, how many you want to grab)
Samplin Method Types?
SRS, stratified, clustered, systematic, multistage, convenience, voluntary
What are the two types of observatinal studies?
Retrospective, and Prospective
What do observational studies and experiments have in common?
In both, you are making OBSERVATIONS.. recording data… doing statistical analysis…
What is a mutlistage sample?
A sample that combines several sampling methods
What is a quality of SRS that is not a quality of Systematic, Stratified or Clustering?
In an SRS, all groups are possible, and ALL POSSIBLE GROUPS have the same chance of being picked. 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 every nth person)
What is a simple random sample?
A sample where every possible group has the same chance of becoming a part of a sample.
What is difference between subject and experimental unit?
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
What is prospective study?
Prosepctive study is when you study the experimental unit’s present and futrue response variable.
What is response bias? How do you avoid it?
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.
What is retrospective study?
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.
What is sampling error?
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).
