I have now had two posts (both rather heavily read and rather contentiously debated in the comments) on detection probabilities (first post, second post). Whether you have or haven’t read those posts, they were fairly technical (although my goal was to explain technical issues in an accessible way).
Here I want to pull way back up to 10,000 feet and think about the boots on the ground implications. And for a change of pace, I’m not going to argue a viewpoint. I just am going to present a scenario (one I see every semester, one that I know students all over the world face from conversations when I travel) and ask readers via a poll what they would advise this student.
So you are on the committee of a graduate student. This student’s project is to study the species Debatus interminus which may be a candidate for threatened listing (little is really known). The primary goals are: 1) to assess overall occupancy levels of D. interminus and 2) to figure out how occupancy varies with four variables (vegetation height, canopy closure, soil moisture, and presence of its one known predator, Thinking clearus). Obviously these four variables are moderately collinear. Given resources, length of project, accessibility of sites, that the student is the only person able to visit the sites, etc you calculate the student can do exactly 150 visits. Various members of the committee have advised the student that she/he should:
- Scenario A – identify 150 sites across the landscape and visit each site 1 time, then estimate ψ (occupancy), and do a simple logistic regression to give β, a vector of regression coefficients for how ψ varies with your four variables across 150 sites.
- Scenario B – identify 50 sites across the landscape and visit each site 3 times, then develop a simple hierarchical model of detection proabilities so you will estimate ψ (occupancy), p (detection probability), and β, a vector of regression coefficients in a logistic regression for how ψ varies with your four variables at 50 sites.
Would you advise the student to follow scenario A or B? And why? Please take our poll (should take less than 5 minutes). I am really curious what our readership will say (and I care about this poll enough that I’ve taken the time to do it in Google polls so I can cross tab the answers with basic demographics – but don’t worry your anonymity is ensured!)
Depending on level of interest I’ll either post the results in the comments or as a separate post after a few days.
And – as everybody knows – a poll in a blog is not a scientific sample, but it can still be interesting.