For concentrations that cannot be negative, a normal distribution of residuals independent of the predicted value may be inappropriate because the suggested prediction interval could expand to negative values. The normal distribution, however, is frequently used for its computational properties.

Can a lower confidence interval be negative?

A population percentage cannot be less than 0%. If the lower endpoint of a confidence interval for a population percentage is negative, it is completely legitimate to replace the lower endpoint by zero: It does not decrease the confidence level. Similarly, a population percentage cannot be greater than 100%.

What does a negative CI mean?

1 Nov 2010, 15:55. Several questions here : (1) Meaning of a negative CI : A negative confidence lower confidence limit suggests the use of an approximate method for calculating the standard error usually in combination with a small sample size.