Confidence Intervals (Pt. 2)

Homer White, Georgetown College

In Part 2:

Load Packages

Always remember to make sure the necessary packages are loaded:


The t-Statistic

68-95 Rule for Estimation

We say that we are about 95% confident that

\[ \bar{x}-2SE(\bar{x}) < \mu < \bar{x}+2SE(\bar{x}), \]

because before the sample was taken

\[ P(\bar{x}-2SE(\bar{x}) < \mu < \bar{x}+2SE(\bar{x})) \approx 0.95. \]

Some Logic

\[ \bar{x}-2SE(\bar{x}) < \mu < \bar{x}+2SE(\bar{x}) \]

means the same as:

\( \mu \) is within 2 SE's of \( \bar{x} \),

which means the same as

\( \bar{x} \) is within two SE's of \( \mu \).

Logic (Continued)

But this means the same thing as:

\[ -2 < \frac{\bar{x}-\mu}{SE(\bar{x})} < 2. \]


For short, let's define the t-statistic as:

\[ t=\frac{\bar{x}-\mu}{SE(\bar{x})}. \]

  • This is the “t” in ttestGC().
  • \( t \) says how many SEs \( \bar{x} \) is above/below the population mean \( \mu \).
  • So \( t \) has “\( z \)-score style”!

Logic (Concluded)

Then all along we were saying that before the sample is taken:

\[ P(-2 < t < 2) \approx 0.95. \]

How good is this approximation? Can we do better?

A Random Variable

\[ t=\frac{\bar{x}-\mu}{SE(\bar{x})}, \\ SE(\bar{x})=\frac{s}{\sqrt{n}}. \]

So \( t \) depends on:

  • \( \bar{x} \) and \( s \), which depend on …
  • the sample, which depends on …
  • chance!

So \( t \) is a random variable!

The t-Curves

Statistical Theory Says:


  • you take a random sample from a population and
  • the population in perfectly normal and
  • your sample is of size \( n \)


  • probabilities for \( t \) are given by a t-density curve with \( n-1 \) degrees of freedom.

What are t-Curves?

  • There is a \( t \)-curve for each degree of freedom \( df = 1,2,3,\ldots \)
  • They are symmetric and centered around 0
  • They have fatter tails than the standard normal curve does
  • But the bigger the degree of freedom is, the more the \( t \)-curve resembles the standard normal curve


To find probabilities for \( t \)-random variables, use ptGC().

Example: Say that you are going to take a SRS of size \( n=40 \) from a population. What is

\[ P(-2 < t <2)? \]


plot of chunk unnamed-chunk-6

[1] 0.9475

At size \( n=40 \), rough 95% intervals are not so bad!


Example: Say that you are going to take a SRS of size \( n=4 \) from a population. What is

\[ P(-2 < t <2)? \]


plot of chunk unnamed-chunk-8

[1] 0.8607

At size \( n=4 \), rough 95% intervals are not reliable!

Finding Multipliers


Our rough 95%-confidence intervals for are of the form:

\[ \bar{x} \pm 2SE(\bar{x}) \]

or more generally:

\[ \textbf{estimator} \pm 2SE(\textbf{estimator}) \]

The 2 is a multiplier. It makes the interval a rough 95%-confidence interval.


We used 1 as a multiplier to make rough 68%-confidence intervals:

\[ \bar{x} \pm SE(\bar{x}) \]

We used 3 as a multiplier to make rough 99.7%-confidence intervals:

\[ \bar{x} \pm 3SE(\bar{x}) \]


  • ttestGC() also uses multipliers to make its confidence intervals.
  • If the population is exactly normal, then they yield exactly the advertised level of confidence!

    How does R find the multipliers?


Say that:

  • you have taken a random sample of size \( n=16 \) from a population;
  • you know the population is normally distributed,
  • you don't know \( \mu \) or \( \sigma \);
  • you want to make a 95%-confidence interval for \( \mu \).


Your interval will look like:

\[ \bar{x} \pm t^*SE(\bar{x}), \]

where \( t^* \) is the multiplier for a 95%-confidence interval for \( \mu \), at sample size \( n=16 \).

We want

\[ P(-t^* < t < t^*) = 0.95. \]

plot of chunk unnamed-chunk-9

[1] 0.95

Looks like \( t^* \) should be about 2.13145.

What R Computes

So at sample size \( n=16 \), R computes a 95%-confidence interval using the formula:

\[ \bar{x} \pm 2.13145 \times SE(\bar{x}). \]

The multiplier depends on:

  • the sample size \( n \)
  • the desired level of confidence


If the population is not exactly normal

  • then R's formula for confidence intervals is not exactly correct!
  • (but as sample size \( n \) increases, it is closer and closer to correct)

The Importance of Safety Checks

Truth in Advertising

Suppose you make intervals at a certain level of confidence, say 95%.

  • For your method to be reliable, your intervals should contain \( \mu \) 95% of the time, in repeated sampling.
  • If the population is normal and you took a random sample, this will happen!
  • (No matter what the sample size is!)

What if ...

… the population is not normal?

Then your intervals are only approximately 95%-confidence intervals.

  • At very large sample sizes, this is not a problem (CLT).
  • But at smaller sample sizes, you must have some reason to believe that the population is not “too far” from normal.

See for Yourself




Safety Check for One Mean

  • At smaller sample sizes (\( n < 30 \), say), make a histogram or boxplot of the sample.
  • If you see much skewness, be worried.
  • If you see outliers, be worried.
  • The smaller the sample size, the more the skewness/outliers should worry you.


The other part of the safety check is:

Did we take a random sample from the population?

If our sample was not random, no amount of clever proability theory will help us make reliable confidence intervals.