To determine the confidence interval of a proportion estimated from a simple random sampling, the standard error (SE) of the proportion must be firstly calculated, based on the following equation:

Where:

p | The prevalence estimated from simple random sampling. |

n | The number of individuals included in the sample from which the prevalence was estimated. |

f | The sampling fraction, is the ratio between sample size and population size (n/N). Its effect is important only when its value is ≥ 0.1. |

The confidence interval is calculated as follows:

Where:

p | The prevalence estimated from simple random sampling. |

z | The value obtained from the standard normal distribution. To each value of confidence there is a correspondent value of z. The levels of confidence more frequently used in biological studies are 90%, 95% and 99%. The values of z correspondent to them are 1.64, 1.96, and 2.58 respectively. |

SE | The standard error of the proportion. |

Instituto Nacional de Tecnología Agropecuaria |
EpiCentre, IVABS, Massey University |