Counting individuals or estimating percentage cover precisely can be slow, so ecologists sometimes use a quick subjective abundance scale. The most common is the ACFOR scale, in which each species in a quadrat or area is given a letter:A = Abundant
C = Common
F = Frequent
O = Occasional
R = RareSome versions add D for Dominant at the top (the DAFOR scale), and many surveys agree a band of percentage cover for each letter, with A the highest and R the lowest.Advantages:
- Quick and easy, so a large area can be surveyed in a short time.
- No counting of individuals is needed, so it works for plants such as grasses that are hard to tell apart.Disadvantages:
- It is subjective: two people may give the same quadrat different ratings, so results are less reproducible.
- It produces ordinal (ranked) data rather than measurements, so a mean cannot be calculated. The letters can be converted to numbers for comparison, for example A = 5, C = 4, F = 3, O = 2 and R = 1, but this does not turn them into true measurements.
- Large, conspicuous species tend to be over-rated and small ones under-rated.To reduce subjectivity, the same person should make all the ratings, or everyone should agree the cover band for each letter before starting.ACFOR is most useful for a first survey of a site, before more detailed quantitative sampling.
C = Common
F = Frequent
O = Occasional
R = RareSome versions add D for Dominant at the top (the DAFOR scale), and many surveys agree a band of percentage cover for each letter, with A the highest and R the lowest.Advantages:
- Quick and easy, so a large area can be surveyed in a short time.
- No counting of individuals is needed, so it works for plants such as grasses that are hard to tell apart.Disadvantages:
- It is subjective: two people may give the same quadrat different ratings, so results are less reproducible.
- It produces ordinal (ranked) data rather than measurements, so a mean cannot be calculated. The letters can be converted to numbers for comparison, for example A = 5, C = 4, F = 3, O = 2 and R = 1, but this does not turn them into true measurements.
- Large, conspicuous species tend to be over-rated and small ones under-rated.To reduce subjectivity, the same person should make all the ratings, or everyone should agree the cover band for each letter before starting.ACFOR is most useful for a first survey of a site, before more detailed quantitative sampling.