Primary Source Documentation
Any document written or recorded by, or for a person who was living at the time and personally involved with the transaction can be considered Primary Evidence. This includes (but isn't limited to) such items as: Wills, Deeds, Marriage bonds, marriage records, Birth records, Census entries (see my additional commentary on census records), Family Bibles, Church Registers, and other such records.
In some cases, the records listed above can't be considered Primary evidence but secondary instead. When possible, all the data I present has been obtained via Primary Evidence. However, that isn't always the case.
Sometimes, primary evidence can be incorrect, or contain minor errors, but this is rare, but possible.
Bible Records - Reliability varies. It depends on whether the entries were made as they occurred, or were they filled in days, months or years later from memory? Often there is no way to tell. When a child would marry they would often get a new bible and would copy the family record out of their parents bible. This process could introduce errors to the record, or additions that were in the original record.
Birth Certificates - Generally very accurate for the mother and father are usually there to supply the information to the Doctor or person making the record. I have seen errors on Birth Certificates but they are usually pretty accurate.
Census Records - Reliability varies greatly depending on the dedication of the census taker or the knowledge of the family member supplying him with the data. I have a page specifically on this subject. Click the link above to find out more.
Church Records - Baptisms, Christenings, marriages, deaths and burials were often recorded by church officials. These vary in accuracy as much as Bible records do, for were they written down as they happened, or compiled later?
Death Certificates - Generally accurate regarding death related information (time, place, cause of death, etc.) as it is usually written by the Doctor. However, the data regarding the deceased's birth date, birth place, parents names, etc, can often be inaccurate depending on the knowledge of the person supplying the data (usually listed as the "Informant"). Often a person who should have accurate data supplies wrong data. This can often be attributed to the stress of the situation, for they have just lost a loved one. On the other hand the informant often just doesn't know the information so it is left blank, or worse yet they guess and get it wrong. I've seen several examples of informants who should have known full well the correct names of the parents of the deceased, yet it was still given wrong on the death certificate for some reason.
Deeds / Land Records - Generally accurate for these are legal records of much importance to the grantee and the grantor, so care would be made to get the names and land descriptions as accurate as possible. However there is often room for error. The deeds you find in the deed books are not the original deeds, but a copy made by the clerk. The original deed was typically made in two copies, one for the grantor, and one for the grantee. The clerk copied whichever one was presented to him for filing. In the older days they were hand copied, so that certainly left room for error.
Marriage Records - Generally accurate for the groom and bride are usually there to supply any required information.
Tombstones / Grave markers - Tombstones vary in accuracy. The date of death is almost always correct, though many tombstones were placed months or years after the death and when that happens there is room for error. The most common error on a tombstone is the birth date. Usually the month and day are correct, but the year is sometimes off a year or two. Sometimes the name of the deceased is spelled wrong. Remember, just because it is "written in stone," that doesn't mean it is accurate. After all it wasn't God who wrote it.