Clarifying use of sessions and occassions

[RyanBurke]

Good afternoon,

I am a beginner with the use of the DENSITY program, but very hopeful it will be beneficial for my research after coming across it being used successfully in other primatological literature. My current stumbling block is with sessions and occasions, as I am getting an error for the number of occasions I have. My trapping array was 2 sites, with 3 transects parallel to the forest edge at 5, 50 and 600 m from the edge. The GPS points for these traps seem to be working fine. I trapped for 30 days over a two month period, alternating between sites. I trapped 2 species of mouse lemurs in Madagascar, and it is also important for me to differentiate between sex. I have simply set up my capture data as one session, with each day being a different occasion. This is obviously not correct.

I would very much appreciate any advice on how to re-arrange my capture data to best suit my purposes. I am sincerely grateful for your time and consideration in advance. Thank you, and have a good weekend!

Sincerely,

Ryan Burke
University of Toronto

[murray.efford]

Ryan

I can't pin down the occasion problem from your description. DENSITY by default determines the number of occasions (days) from the last positive capture record; maybe you had no capture on the last day. You can fix the number of occasions in Options | Input - Capture data.

In DENSITY you probably should define each site x (distance from road) x sex combination as a 'session' (i.e. differing in the first field of the capture file) and then examine different additive between-session models in which the categorical (factor) session covariates (in Options | ML SECR | Between-session model) group these 'sessions' in different ways. All fairly arcane, I'm afraid. Perhaps you should use 'secr' for this rather than DENSITY. There you would have other options and more natural model-specification via formulae. Either way there's quite a lot to learn!

Murray