For a sample size n of 100 visitors to
Safalra.com, user-agent strings submitted by their browsers showed that 52% of the visitors were using
Gecko based browsers such as
Mozilla FireFox.
Find the 95% confidence interval for the population proportion **P** for users of Gecko based browsers who visit Safalra.com.

- ____________________ Write the sample size n.
- ____________________ Write down the sample success proportion p as a decimal.
- ____________________ Calculate q.
- ____________________ Calculate the standard error of the proportion.
- ____________________ Find the degrees of freedom.
- ____________________ Calculate t-critical t
_{c}for a confidence level c of 0.95. - ____________________ Calculate the margin of error E for the proportion.
- Write out the 95% confidence interval for the population proportion:

p(___________ ≤**P**≤ ___________) = 0.95 - ____________________ Visitors using the Microsoft Internet Explorer were 40% of the sample. Is the Gecko based browser usage in the population statistically significantly greater than the Microsoft Internet Explorer usage?

Formulas are written for OpenOffice.org Calc. Replace semi-colons with commas for Excel.

Confidence interval statistics for proportions | |||
---|---|---|---|

Statistic or Parameter | Symbol | Equations | OpenOffice |

Number of successes or desired results in a sample | r | ||

Proportion of successes or desired result in a sample | p | r ÷ n | =r/n |

Proportion of non-successes, not the desired, or alternate result in a sample | q | 1 − p | =1-p |

Standard error of a proportion p | σ_{p} |
=SQRT(p*q/n) | |

Margin of error E for a proportion p | E | =TINV(1-c;n-1)*SQRT(p*q/n) | |

Calculate a confidence interval for a population proportion P from the sample proportion p and the margin of error E for the mean. |
p − E ≤ P ≤ p + E |