If I understand your situation correctly, you may have both left and right
censoring. If a substantial portion of the data is censored, you don't want
to just leave it out because it will, as you said, bias the results. Wayne
Nelson's textbok on Life Data covers this quite well. The book is aimed at
engineers, not statisticians, so it's very clear. I don't remember the
exact title, but it's published by Wiley.
-------------------------------------------
Emil M Friedman, PhD
2304 Richmond Road
Beachwood, OH 44122
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216-591-1750 (voice)
775-249-6744 (fax)
----- Original Message -----
From: "Allard Crispin" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, March 28, 2003 8:12 AM
Subject: Analysis of censored data
> Dear allstat,
>
> I am currently reviewing a simulation model for a client, representing the
> behaviour of a large set of components over time. A component may suffer a
> defect, which is then immediately repaired. From time to time, components
> are replaced for a variety of reasons. An equation for the expected number
> of defects per year has been fitted (least squares regression) using data
> from the last 9 years. Component age is one of the most important
> explanatory variables.
>
> Due to the way the data have been collected, some of the data on component
> ages are missing. The age of every component currently in service is
known.
> In addition, when a defect is repaired, the age of the component has been
> measured and recorded. However, in a case where a component has been
> replaced less than 9 years ago and where there were no defects prior to
> replacement, the age of the original component is not known.
>
> The proportion of components for which the age is unknown ranges from
about
> 1% (last year) to about 10% (9 years ago). These missing data points have
> been excluded from the analysis to fit the equation (however, the equation
> has been normalised so that the total expected number of defects is
correct
> for the whole population). The client believes that the missing ages may
be
> younger than average, and hence some bias may have been introduced.
>
> Any suggestions on ways to approach this problem are gratefully received.
>
> Regards,
>
> Crispin.
>
> Dr Crispin Allard
> Senior Consultant
> QinetiQ Consulting
> Cody Technology Park
> Farnborough GU14 0LX
> Tel: 01252 395312
> Fax: 01252 394109
> Mobile: 07813 832526
> email: [log in to unmask]
>
>
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