Duration field is not listed in sort criteria.

Question

Hi

I am working on a website which has a list of challenges and when a person click on a challenge it displays list of all the participants and their timings. I have added cck field duration to get the timings. But i want to sort the view according to the duration field but duration field is not coming into the sort criteria. Can anyone help please how i can sort according to the duration field or add the duration field to my sort criteria.

Thanks Kanwal