Josipa Kern
University of Zagreb
What is a difference between fiducial limits and confidence limits/intervals?

I am interested in the interpretation
Topics

Confidence Intervals
Statistics/epidemiology
Jul 26, 2013
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All Answers (4)
Perumal Vanamail
Perumal Vanamail · All India Institute of Medical Sciences

Confidence limits (95% or 99%) are calculated either for mean and proportion. In either case the underlying distribution is Normal when sample size is adequately large. Fiducial limts is applicable only in the case of lethal dose required for 50% mortality(LD50) or 90% mortality (LD90). The underlying distribution is logistic growth or S-shaped curve. Even though we transform the data (% values into probit and dose values into logdose) to fit a linear regression equation (Y(probit)=a+b*logdose), the upper limit of conventional confidence limits may be beyond the value (say 110%), which is not true in the case of logistic growth. In such situations Fiducial limits is more appropriate than confidence limits.

Dr.P.Vanamail
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Perumal Vanamail

Perumal Vanamail · All India Institute of Medical Sciences

If we calculate 95% confidence limits for mean or proportion, the interpretation is that we can be 95% confident that the true mean or proportion at population level will lie between lower limit and upper limit of confidence limits. Therefore, the more samples will give us narrow margin of confidence limits. In the similar way, when we calculate 95% fiducial limits for LD50, the interpretation is that with 95% confident the required lethal dose to achieve 50% mortality in the study population species will be within the lower limit and upper limit.

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