Confidence Interval

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Confidence Interval

• Proportions

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Remember

• Point estimate is different every time since it
  depends on the sample selected.
• In choosing a confidence interval, it is not a
  single point but rather an interval of reasonable
  values.

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Confidence Interval

• Interval of reasonable values for the
  characteristic chosen with a degree of
  confidence.
• The value of the population characteristic will
  likely be between the upper and lower bounds.

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Confidence Level

• This represents the success rate of the method
  used to construct the interval.
• If I repeat the process over over,
  approximately this of the intervals will
  contain the true parameter.

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Remember for sampling distributions of
proportions.

• Its approximately normal if

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So, if I want to be 95 confident that my
interval contains the true proportion, how many
deviations would I be from the mean proportion?
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So whats the critical z value that corresponds
to a 98 confidence.
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So whats the formula for a Confidence Interval?
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PANIC!

• P Parameter (tell what we are estimating)
• A Assumptions
• N Name the Formula
• I Interval (calculate it)
• C - Conclusion

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When 320 college students were surveyed, 125 said
they own their car. Construct a 90 confidence
interval for the proportion of college students
who own their car.
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Explain what the 90 confidence level means.

• If I repeat this process over over, 90 of the
  intervals found will contain the true population
  proportion of households that own at least one
  gun.

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A survey of 2450 golfers showed that 281 of them
are left handed. Construct a 90 and 98
confidence level.
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Homework

• Worksheet