Ordinal and nominal data are nonmetric, which means that they have no absolute unit values. Ordinal data are consecutive numbers. Ordinal scales are very common in field ecology, when there are many potentially important factors to measure but no possible way to measure them all absolutely. Various scales are used that can still be used as quantitative values. Relative soil moisture, and perhaps classes of percentage clay would be examples of these.
Nominal data are categorical data.
A species name or community type are obvious examples of these. Nominal data can be very powerful, and the techniques that we will discuss in this class can do a lot with nominal data. That's actually OK, as long as we're cognizant of what we're doing and how it might influence our results. We'll talk a lot more about this when we get to the section of the course on ordination . But look at the first line - a list of species and their abundances.