• A mathematical test to determine association between two species.
• Two main types
  • Testing for independence or association
  • Testing goodness of fit

Both types use a contingency table to compare observed vs. expected results.

• The expected numbers are derived with the assumption of independence.
• Testing goodness of fit might use a model like a ratio for expected numbers.

• 🤔 Hypotheses

  • H0: Two species distribute independently (like strangers at a party).
  • H1: They're associated (like best friends sticking together).

• 📝 Contingency Table

  • Contains observed frequencies.
  • For our example: number of areas containing or not containing the two species.
  • A quick check: Row totals and column totals should yield the same grand total.

• 🔢 Expected Frequencies Calculation

  • Assuming species distribute independently.
  • Use this magic formula
    • Expected frequency = row total × column total

• Scene: Caer Caradoc hill, England.
  • Grazed by sheep 🐑 and crossed by walkers 🚶.
  • Has raised patches with heather (Calluna vulgaris).
  • Observation: Moss species, Rhytidiadelphus squarrosus, seems to be buddies with the heather patches.
• Survey: 100 random samples taken.
  • Question: Is the moss truly associated with the heather?