Practical advice

David L Miller & Jason J Roberts

Real survey data is messy

Collecting and analysing data in the real world

  • We've talked a lot about models
  • We've also talked about assumptions
  • Our example is relatively well-behaved
  • What can we do about all the nasty real world stuff?

Some days...

2 fire emoji, a computer emoji, 2 fire emoji

Aims

  • Here we want to cover common questions
  • Not definitive answers
  • Some guidance on where to look for answers

What should my sample size be?

What do we mean by "sample size"?

  • Number of animal (groups) recorded
    • detection function
  • Number of segments
    • spatial model
  • Number of segments with observations
    • spatial model

Re-frame

How would we know when we have enough samples?

  • We don't
  • Heavily context-dependent
  • Go back to assumptions

"How much data?"

plot of chunk df-obs

Pilot studies and "you get what you pay for"

  • Designing surveys is hard

  • Designing surveys is essential

  • Better to fail one season than fail for 5, 10 years

  • Get information early, get it cheap

    • Inform design from a pilot study

Avoiding rules of thumb

  • Think about assumptions
    • Detection function
    • Spatial model
  • Think about design
    • Spatial coverage
    • Covariate coverage

Coverage

plot of chunk coverage

Disappointment

  • Sometimes you don't have enough data
  • Or, enough coverage
  • Or, the right covariates


    Sometimes, you can't build a spatial model

"Which of X, Y, Z is correct?"

Alternatives problem

  • When faced with options, try them all.
  • Where does the sensitivity lie?
  • What's really going on?

"How big should our segments be?"

Segment size

  • If you think it's an issue test it
  • Resolution of covariates also important
  • Maybe species-/domain-dependent?

"Is our model right?"

Model validation

  • Some variety of cross-validation
  • Temporal replication
    • Leave out 1 year, fit to others, predict, assess
  • Spatial “pseudo-jackknife”
    • Leave out every \( n^{th} \) segment, refit, …
    • (Maybe leave out 2, 3 etc…)

Getting help

Resources