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Assessment of Fuel Economy Technologies for Light-Duty Vehicles
The committee did not address what specific probability distribution the uncertainty about fuel consumption and cost impacts might take. However, if one assumes they follow a normal distribution, then the ratio of a 90 percent confidence interval to an 81 percent confidence interval would be approximately 1.64/1.31 = 1.25. Thus, an appropriately rough adjustment factor to convert the individual confidence intervals to a joint confidence interval of 90 percent would widen them by about 25 percent.
Goodman, L.A. 1962. The variance of a product of K random variables. Journal of the American Statistical Association 57(297):54-60.