The U.S. Federal Census is a valuable tool for researching your family history, especially for those who don't know much about their ancestors beyond their grandparents or great-grandparents. Finding one of them on the census usually leads to the discovery of their parents or guardians names.
A common problem with using the census is the discrepancies found. It is common to see minor variances or errors in the ages of the household members from census to census. A woman who was 30 in 1900, for example, should have been 40 in 1910, but I've seen many cases where her age was given as 37, for example, in 1910, instead of 40. Was it a mathematical error? Did someone else who wasn't quite sure give the census taker the information? Or was she skimming years to avoid revealing her true age? This is not uncommon! So we have to wonder how reliable the informant was. Knowing who the informant was, would help, but unfortunately, there was usually no indication of who the informant was, until the 1940 census.
When the 1940 census was taken, a small notation was made to let us know. A circled "x" beside the name indicated who the informant was. The mark, in a way, gives us a glimpse of the scene. Given the example below, I can picture my great-grandmother giving the census taker the information, either recalling or calculating the ages of the seven children in the home. The ages happen to all be accurate.
In most cases, the mother would be the most reliable source of information, having given birth to all the children. A child, of course, would be less reliable. This is just one small way to help determine whether or not the information is reliable.
The United States 1940 Federal Census can be searched free of charge on FamilySearch at: https://www.familysearch.org/search/collection/2000219