Probability Plot

by JLee   Last Updated September 11, 2019 18:19 PM - source

I am trying to reproduce a plot from a paper. The plot is a Weibull probability plot and is used to illustrate why a Weibull distribution is appropriate for modelling failure times.

The plot is shown below.Weibull Probability Plot

In order to reproduce this image. I assumed that the failure times followed a Weibull distribution. I obtained the ML estimates and corresponding 95% CIs for the parameters (the values obtained matched the values obtained by the authors).

I then calculated the probability of failing for times between 0 and 70 using the Weibull CDF with the ML estimates, and with the lower and upper points of the CIs.

I then plotted the actual data points (proportion of units failing over time) and the probability of failing using the three pairs of parameters described above. My plot is shown below.

My plot

My plot does not look like the plot the authors produced. Have I misunderstood what is meant my a probability plot?

I understand that the axes chosen by the author are different to my axis, but if you focus on the x-axis between 0 and 70, all of their data points seem to be inside the CI. This is not the case for my plot.

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