A confidence interval consists of two confidence limits. In some cases, Prism can only determine one confidence limit and reports the other limit as "???".

If the upper limit is shown this way, you can say with 95% (or whatever confidence level you chose) that the parameter is greater than the lower limit, but cannot say anything about how large the parameter could be.

If the lower limit is shown this way, you can say with 95% (or whatever confidence level you chose) that the parameter is smaller than the upper limit, but cannot say anything about how small the parameter could be.

It's important to note that Prism reports "???" for one or both confidence limits when the calculation to determine these values fails for any reason. Thus, the most appropriate interpretation of this result is that the confidence limit replaced by "???" is undefined.