Confidence Intervals (Pt 4)

Rebekah Robinson, Georgetown College

In Part 4:

Load Packages

Always remember to make sure the necessary packages are loaded:



Width of Confidence Intervals

The width of confidence intervals are affected by:

  • sample size
  • confidence level

Check it out:



In general,

  • larger sample sizes produce more narrow confidence intervals.

  • higher levels of confidence produce wider intervals.

Sampling Bias

  • Confidence intervals are based on samples, so are subject to any type of bias that can occur in the sampling process.

  • Confidence intervals based on a poorly drawn sample are poor inferential tools.


A telephone survey using land line phone numbers was used to poll people on their intended vote for an upcoming election. Researchers intend to use a confidence interval to report the proportion of the population that plan to vote for the Republican candidate.

Can we find any problems with the design of this study?


This study is subject to selection bias because a large part of the population will have been excluded from this survey.

People who own only cell phones will be under-represented. These people also tend to be:

  • young
  • liberal, etc.

Hence the sample probably overestimates the proportion of Republicans in the population.

Making a confidence interval for \( p \) does nothing to correct this bias!!

Incorrect Interpretations


Research Question

What is the average height for folks from the imagpop population?

For a sample of size 50, the 95% confidence interval is

\[ 66.3 \text{ to } 68.3 \text{ inches}. \]

Correct Interpretations

  • We are 95% confidence that the average height of the population lies somewhere in the interval (66.3, 68.3).

  • The interval (66.3, 68.3) gives a range of believable values for the average height of the population.

  • If we were to repeatedly take samples of size 50 and construct 95% confidence intervals for each sample, about 95% of those intervals would contain the true mean height of the population and about 5% would not.

Incorrect Interpretations

  • There is a 95% chance that the true population mean height lies somewhere in the interval (66.3, 68.3).

  • The probability of the average population height being between 66.3 and 68.3 inches is 0.95.

  • About 95% of the folks in the population have an average height between 66.3 and 68.3 inches.

  • The interval (66.3, 68.3) gives a range of plausible values for the average height of the folks in the sample.