However, that isn't always the case. Sometimes, primary evidence can be incorrect. Any document written by a third party or shortly after the decease of the person in question, should be considered Secondary Evidence. Secondary Evidence is often reliable, but shouldn't always be assumed correct. Census records can often be secondary evidence, since there is usually no way to know if the data given to the census taker was given by a family member or a neighbor, or if the data was reported accurately.
Secondary evidence is often quite accurate, but has a higher error rate that primary evidence.
This often leads to the same incorrect data being passed around again and again. Family Histories are a good starting place, but they can't be considered evidence unless fully documented. This applies to these web pages as well. Don't assume that everything listed is correct. I have several cases of data that is well documented in Primary Records, but was once published incorrectly in some family history, and everyone keeps trying to correct me by referring me to the published family history.