[R] combining same-day lab measurements with 'apply'

Another request for help implementing the 'apply' functions to avoid a loop structure... I am working with a data set that includes lab measurements taken at different dates for the subjects, with some subjects having more results than others. I would like to average lab results for each subject

[R] error: bad value ???

in the same session and used 'apply' in other sessions without this problem. I had a much harder time searching for this in the archives, so I don't know if this is a related or a separate issue. Any ideas building on those already expressed in the other posts? Did either of the