From a genealogical perspective, census returns are perhaps the most valuable resource available. As genealogists, we study families and the census returns show us the population of areas by family. I have found that when I hit a 'dry' time in researching, one of those occasions when I just don't seem to be able to find the document I want or really know where to go next, examining census returns usually provides the clue that breaks the 'brick wall' and leads to a wealth of additional information. As an example, I experienced this when, in one of those 'dry' times, I decided to track my Gaull ancestor's movements through Aberdeenshire, Scotland. As I examined each census return in chronological order (from earliest to most recent), I discovered the family included a person in 1901 who I had never heard of previously. This individual, listed as the grandson of my 2nd great grandfather, lead me to the discovery of "Uncle Disney" (more on him at a later date).
Without a doubt, birth and death records are primary sources and I love finding marriage registrations as they can typically contain a wealth of interesting tidbits of information about the couple, their parents, their residence, and their friends or siblings who may have served as their witnesses. While census returns are considered by many to be a secondary source of information, there is still room for debate on this topic.
When I find an ancestral family in a census, unlike other documents, I am given the opportunity to see the whole family and some of its activities like occupations, who was going to school, who the neighbours were, perhaps what type of house they lived in, and sometimes what the annual income was, etc. It is also important to check to see if there was a page two of the census return (check the page before and the page after the return of the page the family appears on) because if the family lived on a farm as many ancestors did not too long ago, the frequently completed agricultural census will tell you what crops they were growing and what type of livestock they kept.
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