I am looking for some advice on the use of NULL and NA in my data set. Very specifically, I have fishery data from independent observers. The data consists of patrol information with variables such as patrol_distance, patrol_time, etc, and fishing information with variables such as angler_count, fish_count, etc. My question is: what do I assign to cases where no anglers were counted, NULL or NA. I will need to do computational summaries on these variables and later export to a database (MS Access).
I have already done some reading on the use of NULL and NA, but just require clarification.