What are correction factors?
Issues of indirect consumption and "bottom up" versus "top down" footprint calculations.
The ecological footprint is flexible enough to be calculated for any number of individuals, including whole countries. Wackernagel et al. (1996) have compiled footprints for 152 nations of the world. The household footprint that you are using for this project has been used to estimate the footprint of the entire United States.
The national footprint calculation for the US is very complete. In the national calculation, the total US footprint is divided into subfootprints of Food, Housing, Transport, and Goods and Services, in a table called the "Land use consumption matrix." Because data on total food production and export, housing, transportation, energy, and commerce are readily available from sources like the US Departments of Agriculture, Housing and Urban Development, Transportation, Energy, and Commerce, it is relatively simple to capture most all the resource and energy flows in the economy, in the aggregate, including waste. This is called the "top-down" approach because it uses aggregate data to estimate an aggregate footprint for all of the people in the US. These aggregate data can be plugged directly into the spreadsheet, and an aggregate footprint for the US is calculated.
It is important to note that this kind of aggregate accounting is capable of quantifying "indirect consumption" that is difficult to quantify for individuals. For example, consider the following question: How much energy does it take to operate a car? Besides the obvious things that come to mind, like the amount of gasoline you put into a tank, there are many indirect energy requirements that buying, owning, and maintaining a car entails (Herendeen 1998). These include the energy to find, extract and refine petroleum. They also include energy to build, maintain, and operate car manufacturing plants, retail dealers, and maintenance shops. These indirect uses of energy can total an additional 63% of the energy provided by the gasoline alone (Herendeen 1998). Thus, our consumption of energy for our automobiles extends far beyond the direct use of gasoline. Top-down national footprint calculations, because they account for all of the material and energy flows in the US, are good at quantifying indirect energy use.
The household footprint that you are using is relatively complete for a household consumer, and it represents a "bottom-up" approach. In contrast with the national footprint calculated with aggregate US data, it misses some of the indirect flows of resources and energy, because it is extremely difficult for you to account for these. In this respect, the individual footprint spreadsheet underestimates total material and energy consumption and waste production in the US. Put another way, if all 280 million citizens of the US were to complete individual spreadsheets and then we were to add up these individual footprints, the cumulative footprint of the US would be less than the footprint based on the aggregate data. Thus, there needs to be a way to correct this discrepancy between the top-down and bottom-up approach.
To do this, Wackernagel et al (2000) filled out the household questionnaire using what we know to be the average consumption amounts for the different categories. They compared the footprint subtotals, or the Food, Housing, Transport, and Goods and Services footprints, derived from using this household questionnaire, with the sub-footprints from the national calculation. They used the ratios between these categories to correct the household footprint. These ratios, or correction factors, shown in cells B137-I143 calibrate the household spreadsheet so that it accounts for indirect uses of materials and energy. These factors aren't perfect by any means, but it's the best method available for correcting the footprint to capture information that may be missed with the bottom up approach.
Information Credit: This information was provided by Diana Deumling, Redefining Progress