causes of error in sampling. sampling error sampling error is error caused by the way you chose your...
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Sampling Error Random Sampling Error Random Sampling Error – You sample may randomly have a higher or lower percentage of females, college educated people, Hispanics, etc. than what what is found in your population. – Causes Variation How do you reduce? – Choose a larger sample size.TRANSCRIPT
Causes of Error in Sampling
Sampling Error• Sampling error is error caused by the way you
chose your sample– Volunteer Sampling & Convenience Sampling– Causes Bias
• How do you reduce?– Use randomness in choosing
your sample
Sampling ErrorRandom Sampling Error
• Random Sampling Error– You sample may randomly have a higher or lower
percentage of females, college educated people, Hispanics, etc. than what what is found in your population.
– Causes Variation• How do you reduce?– Choose a larger sample size.
Sampling ErrorRandom Sampling Error
• From the bell curve, 95% of the data lie within + ______ standard deviations
• This means that we are 95% confident that the true value is within 2 standard deviations of our measured value.
Std. Dev. for Categorical Variables
2
Sampling ErrorRandom Sampling Error
• If you change the 2 you can find other confidence intervals.– Example: Using 3 yields 99.7% confidence interval– We’ll mostly stick to 95% confidence
• This only finds the expected error from randomness… variability.
• Does not account for bias.
Sampling ErrorUndercoverage
• Undercoverage is when some groups in the population are left out of the process of choosing the sample.
• What groups CAN’T be contacted by a survey where you call random people and ask their opinion?– Amish– Homeless– People serving overseas– Prison Inmates– Unlisted numbers & other people without phones
Sampling Error:Undercoverage
• Undercoverage causes… • How do you reduce/prevent undercoverage– Census (poll everyone)– Reduce bias in your sampling (avoid convenience
& volunteer sampling)– Stratified Sample: Split the population in groups
and then sample each group.• Example: Instead of randomly selection 100 people,
randomly select 50 men and 50 women
bias
Nonsampling Error:Processing Error
• Examples– Enter a number wrong in Excel– Math Error
• How do you reduce/prevent– Double Check your Work– Don’t Rush
• Causes Variation
Nonsampling Error:Response Error
• Response Error – given an incorrect response• Why would someone do that?– Lie• How much do you weigh?• Have you ever used drugs?
– Remember Incorrectly• How many minutes have you watched TV this week?• Where exactly were you at 3:30 PM last Saturday?
– Vague or Confusing Question• How many windows do you have? Do door windows
count?
Nonsampling Error:Response Error
• Lying causes bias• How can we reduce lying?– Confidential: the interviewer promises their name
won’t be released with the results– Anonymous: even the interviewer doesn’t know
which response sheet corresponds to which person
– Study them without them knowing, as with the handwashing homework problem• Is this ethical?
Nonsampling Error:Response Error
• How can we reduce memory errors?
Nonsampling Error:Response Error
• Confusing questions cause bias if they favor one answer over another. They cause variability if they are just confusing in general, but don’t favor a particular outcome
• How can we reduce people from misunderstanding the question?– Clear, careful and extremely specific wording
Nonsampling Error:Response Error: Question Wording
• 13 % of Americans think we are spending too much on “assistance to the poor,” but 44% think we are spending too much on “welfare”
• A poll in Scotland showed that 51% would vote in favor for “independence for Scotland,” but 34% would vote in favor for “an independent Scotland separate from the United Kingdom.”
• Assistance & Independence are positive words while Welfare & Separate are negative words
Wording Questions• Loaded questions cause bias– Do you favor banning private ownership of handguns
in order to reduce the rate of violent crime?– George Bush: great president or greatest president?
– Do you support our president?– Do you agree with all of Obama’s policies?– Do you approve or disapprove of the way Barack
Obama is handling his job as president?
Wording Questions: Vague• How many windows are in your house is a
simple enough question…– Does this room have 2 or 12 windows on the wall?– Do the windows on the door count?
• If you’re not specific, everyone is going to interpret the question differently adding a lurking variable and variability to your data.
Wording Questions
• Open vs. Closed Questions– Rate Obama’s performance on a scale of 1 to 10
vs. what do you think about Obama’s job as president?
– Limiting people’s options can cause bias– Limiting people’s options can reduce people
misunderstanding the question– Closed Questions are easier to analyze
Non Response
• They refuse to answer your survey. They hang up on you, slam the door in your face or just politely say no.
Nonadherer
• A nonadherer is a generic term for someone who doesn’t follow directions for whatever reason.– Example: They forgot to take the experimental pill
every 6 hours.– Example: They lied on their survey form
• How would you reduce?
Dropouts
• Sometimes your study the same group over several days, weeks, months or even years. Dropouts are people who start doing the experiment, but then stop before it is complete.