Notes 4.1.notebook
October 26, 2011
Chapter 4: Designing Studies
Activity: See no evil, hear no evil? (pg 206)
"I hear and I forget. I see and I remember. I do and I understand." - Confucius
Determine class order - alphabetical by first name.
In pairs...
Student A: Study card quietly for 30 seconds. Recite the
alphabet out loud. Tell Student B what you think the numbers
on the card are. Student B will record how many correct.
Student B: Read the pairs of numbers aloud 3 times slowly.
Recite the alphabet aloud. Tell Student A what you think the
numbers on the card are. Student A will record number
correct.
Parallel dotplots in Fathom.
Sampling
We choose a sample when we cannot collect data for
the whole population.
EX) Sampling Students and Soda
(a) The student government at a high school surveys 100 of the
students at the school to get their opinions about a change to
the bell schedule.
Population:
Sample:
(b) The quality control manager at a bottling company selects a
sample of 10 cans from the production line every hour to see if
the volume of the soda is within acceptable limits.
Population:
Sample:
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Notes 4.1.notebook
October 26, 2011
Surveys
http://www.youtube.com/watch?v=KpUjMoXo800
EX) Unemployment Rates
Current Population Survey (CPS) contacts about 60,000 households
each month to determine information such as unemployment rates.
EX) Consumer Confidence
Each month the Conference Board takes a sample of 5000 households
and asks them five questions about their appraisal of current business
and employment conditions and expectations.
Conducting a Sample Survey
1. Identify the population of interest.
2. Identify response variable (what you want to know).
3. Design a procedure to collect representative data.
How NOT to sample
CONVENIENCE SAMPLE:
Sampling those who are easiest to reach.
Ex) Pop of interest: Student body
Sampling: Only students in your classes.
Favors:__________________________
Ex) Pop of interest: All Americans
Sampling: Standing in mall, asking anyone who will stop.
Favors: _________________________
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Notes 4.1.notebook
October 26, 2011
How NOT to sample (part 2)
VOLUNTARY RESPONSE SAMPLE:
Sampling those who CHOOSE to respond.
Ex) Pop of interest: American voters
Sampling: Call-in survey to news program
Favors:__________________________
BIAS: SYSTEMATICALLY FAVORS CERTAIN OUTCOMES.
Ex) A parent is concerned about the proposed budget cuts to
the athletic program, so she asks spectators at the game if
they agree.
Would her resulting proportion of "No"s be a high or low
estimate?
CYU (pg 211)
Identify the sampling method used. Explain how it could lead to bias.
1. A farmer brings a juice company several crates of oranges each week. A company inspector looks at 10 oranges from the top of each crate before deciding whether to buy all the oranges.
2. The ABC program Nightline once asked whether the UN should continue having its headquarters in the US. Viewers were invited to call one telephone number to respond "Yes" and another for "No." There was a charge for calling either number. More than 186,000 callers responded, and 67% said "No." Is this likely an over or under estimate?
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Notes 4.1.notebook
October 26, 2011
How to sample WELL
SIMPLE RANDOM SAMPLE (SRS):
Randomly chosen in such a way that every possible sample of
the given size has an equal chance of being chosen.
Think back to the activity at the beginning of class. Was
there bias in the process of pairing up students?
Activity: How large is a typical state?
Use the map shown to estimate 5 states of average land area.
(BTW, this is NOT the way to sample well.)
Now, use the table to calculate your the mean land area for
your sample.
(dotplots on Fathom)
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October 26, 2011
Random Digit Table
Use the Random Digit Table to select five states (sets of numbers).
Some rules:
1. Use the same number of digits for each item.
i.e. If going 1 - 50, use 01 for 1, 02 for 2, etc.
2. Know whether or not you can count repeated numbers.
Demo
Going back to the table, calculate your new mean area.
(new dotplot on Fathom)
How do they compare?
Random Number Generator
On Calculator:
Math -> PRB -> RandInt(min, max, how many to choose)
Select 5 states this way...
How does the distribution of our randomly chosen
states compare to the distribution of when we "tried"
to randomly select?
We estimated on the high side...
Random sample
Random sample
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