# Likelihood Ratio Test Example

*This example appears in the Accelerated Life Testing Data Analysis Reference book.*

Consider the following times-to-failure data at three different stress levels.

Stress | 406 K | 416 K | 426 K |
---|---|---|---|

Time Failed (hrs) | 248 | 164 | 92 |

456 | 176 | 105 | |

528 | 289 | 155 | |

731 | 319 | 184 | |

813 | 340 | 219 | |

543 | 235 |

The data set was analyzed using an Arrhenius-Weibull model. The analysis yields:

- [math]\widehat{\beta }=\ 2.965820\,\![/math]

- [math]\widehat{B}=\ 10,679.567542\,\![/math]

- [math]\widehat{C}=\ 2.396615\cdot {{10}^{-9}}\,\![/math]

The assumption of a common [math]\beta \,\![/math] across the different stress levels can be visually assessed by using a probability plot. As you can see in the following plot, the plotted data from the different stress levels seem to be fairly parallel.

A better assessment can be made with the LR test, which can be performed using the Likelihood Ratio Test tool in ALTA. For example, in the following figure, the [math]\beta s\,\![/math] are compared for equality at the 10% level.

The LR test statistic, [math]T\,\![/math], is calculated to be 0.481. Therefore, [math]T=0.481\le 4.605={{\chi }^{2}}(0.9;2),\,\![/math] the [math]{\beta }'\,\![/math] s do not differ significantly at the 10% level. The individual likelihood values for each of the test stresses are shown next.