Life of Incandescent Light Bulbs

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This example uses time-to-failure data from a life test done on incandescent light bulbs. The observed times-to-failure are given in the next table.   

Observed times-to-failure for ten bulbs in hours.
Order Number Hours-to-failure
1 361
2 680
3  721
4 905
5 1010
6  1090
7 1157
8 1330
9 1400
10 1695

Do the following:

  1. Plot the data on a Weibull probability plot and obtain the Weibull model parameters.
  2. Compute the B10 life of the bulbs.


The median ranks  for the  the [math]\displaystyle{ {{j}^{th}} }[/math] failure out of N units is obtained by solving the cumulative binomial equation for [math]\displaystyle{ Z }[/math] . This however requires numerical solution.  Tables of median ranks can be used in lieu of the solution.


Median Rank Tables



represents the rank, or unreliability estimate, for the  failure[15; 16] in the following equation for the cumulative binomial:

[math]\displaystyle{ P=\underset{k=j}{\overset{N}{\mathop \sum }}\,\left( \begin{matrix} N \\ k \\ \end{matrix} \right){{Z}^{k}}{{\left( 1-Z \right)}^{N-k}} }[/math]


where [math]\displaystyle{ N }[/math] is the sample size and [math]\displaystyle{ j }[/math] the order number. The median rank is obtained by solving this equation for [math]\displaystyle{ Z }[/math] at [math]\displaystyle{ P=0.50, }[/math]

[math]\displaystyle{ 0.50=\underset{k=j}{\overset{N}{\mathop \sum }}\,\left( \begin{matrix} N \\ k \\ \end{matrix} \right){{Z}^{k}}{{\left( 1-Z \right)}^{N-k}} }[/math]