Confidence Intervals

Calculation Steps

  1. choose the population statistic
  2. find its sampling distribution
  3. decide on level of confidence
  4. find the confidence limits

Cheat Sheet

Population Statistic Population Distribution Conditions Confidence Interval
μ Normal You know what σ2 is
n is large or small
x¯ is the sample mean
$$(\bar{x}-c\frac{\sigma }{\sqrt{n}},\bar{x}+c\frac{\sigma }{\sqrt{n}})$$
μ Non-normal You know what σ2 is
n is large (at least 30)
x¯ is the sample mean
$$(\bar{x}-c\frac{\sigma }{\sqrt{n}},\bar{x}+c\frac{\sigma }{\sqrt{n}})$$
μ Normal or Non-normal You don’t know what σ2 is
n is large (at least 30)
x¯ is the sample mean
s2 is the sample variance
$$(\bar{x}-c\frac{s}{\sqrt{n}},\bar{x}+c\frac{s}{\sqrt{n}})$$
p Binomial n is large
ps is the sample proportion
qs is 1−ps
$$(p_{s}-c\sqrt{\frac{p_{s}q_{s}}{n}},p_{s}+c\sqrt{\frac{p_{s}q_{s}}{n}})$$
μ Normal or Non-normal You don’t know what σ2 is
n is small (less than 30)
x¯ is the sample mean
s2 is the sample variance
$$(\bar{x}-t(v)\frac{s}{\sqrt{n}},\bar{x}+t(v)\frac{s}{\sqrt{n}})$$
where c is:
Level of Confidence Value of c
90% 1.64
95% 1.96
99% 2.58
and t(ν) is the T-Distribution

interval in general

statistic ± (margin of error)
margin of error = c * (standard deviation of statistic)

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