An Online CNN Poll

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An Online CNN Poll
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Probabilistic Sampling Methods

• Simple Random Sampling
• Each elementary unit in the population has an
  equal chance of being selected for the survey
• Random Digit Dialing
• Pick telephone numbers at random
• Stratified Random Sampling
• Take a random sample from each strata
• Cluster Sampling
• Take a random sample of clusters and survey
  everyone in the cluster

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Non-Probabilistic Sampling Methods

• Systematic Sampling
• Every nth person (whatever) is chosen
• Convenience Sampling
• Respondents who are willing and available are
  selected
• Snowball Sampling
• Previously-identified respondents identify others
  (ask a friend)
• Focus Groups and Expert Panels

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Two Variables, x y
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Sampling Distribution of x
_

• 1. Mean (Expected Value)

• 2. Variance

• 3. Shape
• x has a t distribution if the sample size is
  large

• _

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Central Limit Theorem
Distribution of x
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Exercise 1

• The sign on the elevator says maximum capacity
  2500 pounds or 16 persons. Weights of people who
  use the elevator is normally distributed with
  mean 150 pounds and standard deviation of 20
  pounds. What is the probability that 16 people
  weigh more than 2500 pounds?

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Sampling Distribution of p
_

• 1. Mean (Expected Value)

• 2. Variance

• 3. Shape
• p is normally distributed if the sample size is
  large (np?5 and n(1-p) ?5)

• _

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Exercise 2

• In the population of all statistics students, the
  fraction of students that fall asleep is 0.20. In
  a class of 25, whats the probability that more
  than four fall asleep?

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Sampling Distribution of b (regression coeff.)

• 1. Mean (Expected Value)

• 2. Variance

• Requires homoskedasticity and independent errors

• 3. Shape
• b has a t distribution if the sample size is
  large (CLT for errors)

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Use the t distribution!

• Always need to estimate and
• So distribution of x and b have a t distribution
• Degrees of freedom
• Sample mean case n-1 d.f.
• Note just estimating one parameter (variance)
• Regression case n-k-1 d.f.
• knumber of coefficients excluding the intercept
• the -1 captures the intercept

• _

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t Critical Values