Sources and Reliability
When you "Ask Chris" the advice asks you to include information on sources - and it is surprising how many people totally ignore this advice - which makes my job harder - and my advice is less valuable if your question was based on dubious evidence. The following points should be noted:
All records are likely to contain errors, ambiguities and other faults - and this will include your note and my answers. Hopefully the number of errors are low but it is important to know that a few will always get through.
Apart from some modern records, the original documents were hand-written, with copies also being hand-written. Unless you have had experience of reading such documents you will not realise how unreliable transcripts and indexes can be. The web page My Ancestors in the 1901 Census provides an excellent introduction to the transcription errors which can occur - with the "dubious" handwriting reproduced.
The original document (or a good microfilm of it) is more reliable than any transcript or index, and a family tree will be less accurate that the source documents (and indexes) which were used to draft it.
Information provided by a highly literate person about himself is likely to be more accurate than verbal information provided by someone who was illiterate, and written down by a stranger.
In general the most reliable information is likely to be original documents such as birth certificates, wills, family bibles with contemporary records, etc.. However some people emigrated to escape their past and their "recollections" of their origins can be deliberately misleading. In addition some family bibles may contain information added from memory (or by an amateur genealogist many years after the event) and so may be less reliable.
Official records such as purchased birth, marriages and death certificates - and the official indexes - may be pretty reliable records of some birth, marriage or death, but care must be taken to ensure that it is the right individual (see Right Name, Wrong Body). Such records, while mainly correct contain a vast number of errors. (See A Comedy of Errors). When someone left Hertfordshire, records may be made by officials who know little about the county and additional errors can creep in.
Indexes are worse because there is scope for introducing new errors (omissions, misreads, etc.) in preparation. They also only tell part of the story. This is why you should always check the source document (or its microfilm) to confirm the original data.
Family Trees can be the worst source of reliable information. They rarely record the source of the information and may hide complications. For instance the person who first drafted the tree may have guessed that a baptism for a "John Smith" related to the same person as a "John Henry Smith" who married 25 years later - and the family tree shows the baptism as "John Henry Smith". Modern computer techniques encourage some people to collect as many names as possible without ever bothering to check the sources, often making outrageous guesses. - "Never mind the quality, feel the width"
There is a question you should ask yourself at this point. Do you have any names on your family tree which were accepted unchecked from John who copied them unchecked from Bill who had got some names from a tree that James had drafted and some from a rough draft from William and crudely linked the two together in a devil may care manner. William had been careful and had shown some relationships as "guesses" - but nobody copied the fact that they were guesses ... so everyone thinks they are true - despite the fact that William has now checked and corrected the errors in his draft. Why not read The Dangers of Internet Genealogy and ask how much unchecked and possibly unreliable information you have.
Note added January 2011 - The following paragraph was written before a major upgrade to the familysearch web site - and a few tests suggests that the data is now more accurate. Hovever family trees drawn up before 2011 may well contain contain misleading information from this site.
This brings us to the problem that causes many beginners major headaches - and wastes much of my time. The International Genealogical Index (IGI) on familysearch is an index of people who have retrospectively been accepted into the Church of Latter Day Saints. Some of these entries are extracted straight from the parish registers and are reasonably reliable (which should still, of course, be checked). Other entries come from family trees, generated by members of the church who were more interested in saving a maximum number of souls than in genealogical accuracy. I have come across many cases where there are highly dubious entries - for instance baptisms assigned to particular churches when there are no such entries in the register. The basic rule is to disbelieve anything from the IGI which is not directly from a register extraction until you have established the reliability by checking sources. (see The Limits of Familysearch)
Similar comments can be made about other sources - for instance people often raise queries about the 1881 census transcripts which could easily be solved by looking at microfilms of the original. (see My Ancestors in the 1901 Census). Others raise queries about a birth/baptism without bothering to tell me whether the information comes from a much treasured family bible, a dubious entry from familysearch, or a family tree picked up on the internet whose accuracy is totally unknown.
Please include Source information with all your queries
If the information comes from a site such as Ancestry or FindMyPast please indicate which database the information comes from.
See also I've hit a brick wall ..., Is this the right birth/baptism and How can you be certain about ...