Take a look at the following map of obesity in Canada.  Who is missing from the map? Can you suggest reasons why these groups are missing? Are these gaps related to equity?
There are many reasons why we have so little information about the Territories. It can be costly and time consuming to collect data with small populations. At the same time, when the information we have is about a small number of people, the government does not release it to the public in order to protect people’s privacy. Technical issues can also create challenges for gathering and reporting on data for small populations. Because this particular map is based on the same areas used in the Census, it only includes communities in the Territories that are large enough to allow for comparisons about weight. For these and other reasons, national surveys often do not include residents of the Territories and First Nations Reserves as well as other small populations.
Whatever the reasons for this knowledge gap, it creates the potential for inequity because part of the population is excluded from the analysis. In the case of the Territories, the absence of data is especially troubling because Aboriginal people represent a significant proportion of the population and we know from other sources that obesity rates are much higher among Aboriginal than non-Aboriginal people.
Graph: Prevalence of measured overweight and obesity among non-Aboriginal and Aboriginal adults (18–64 years of age) and children and youth (2–17 years of age), 2004, Canada 
These kinds of limitations in our information about populations crop up all the time and we have to find ways to work with them. It is important to understand how the information available to us on any subject enriches or restricts our knowledge about the populations that concern us.
Sources:  Statistics Canada and Canadian Institute for Health Information 2006.
Figure source: Statistics Canada, 2008, “The Population Ecumene of Canada: Exploring the Past and Present”, Geography Working Paper Series, Catalogue number 92F0138MWE. http://www.statcan.gc.ca/pub/92f0138m/2008003/figures/figure3.3-en.jpg;  Katzmarzyk, P.T. (2008). Obesity and Physical Activity Among Aboriginal Canadians. Obesity, 16, 184–190