The width of each arrow on the SFD Graphic is proportional to the percentage of the population whose excreta contribute to that flow, not volumes of excreta or waste. This is because data on the types of sanitation systems used are normally available via national census data or health surveys, and are based on the number of people. To convert this data to volumes would require a large number of assumptions to be made such as:
- the amount of waste each person produces per day
- the amount of water used to flush the waste in water-based systems
- the amount of leakage in sewers etc.
Typically very little data are available to enable accurate or credible estimates to be made on the basis of volumes. Population percentages are also relatively easy to comprehend for non-technical stakeholders.