The purpose of measurement is to make values more precise, to compare and evaluate one thing against another. Quantitative indicators may include trees, animals, incidence of sickness, sacks of maize, etc. They may include complex ratios or percentages that indicate the incidence of important phenomena. Scales Value measurements are derived from the nature of the thing valued and translated onto a scale. Ordinal scales identify category and rank order.
Interval scales identify rank order and have equal intervals.
Addition and subtraction may be used. Ratio scales identify rank order and interval and have an absolute zero. This allows for more complex mathematical operations. The more complex scales may be mapped onto the simpler but not vice versa. In assessing sustainability we normally use ordinal or interval scales.