The IGI and The Ancestral File

see:
LDS IGI On-line – Family Search

The International Genealogical Index (IGI) is an index created by the Church of Latter Day Saints (LDS). The index is available for viewing at all LDS Family History Centres, on microfiche or CD ROM, and now also available for searching on the Internet on the LDS web pages. It is possible to purchase the IGI on microfiche (in whole county sets only) from the LDS at a very nominal cost. The IGI is certainly the most comprehensive available index of English parish baptisms and marriages available. (Burials are not in the index, except for a few isolated examples).

However:

• The IGI does not cover every parish in England. A great number are not included.
• For those parishes which it does cover, there may be whole periods missing.
• For those which it does cover, there may be omissions of individual records even in the covered periods.
• Some entries in the IGI are from the Bishop's Transcripts and not the original registers, and the BTs themselves are prone to errors and omissions. (Having said that, even the original registers may have entries missing which are in the BTs and vice versa).
• You may find more than one entry in the index for the same event with conflicting information.
• Bear in mind that the IGI is primarily an index of ordinances carried out by LDS members as part of ther religion, rather than an index of parish registers.
• Some entries in the IGI are from submissions by individual LDS members rather than from parish registers, and many of these are very prone to errors. Be very wary of entries which have an "@" symbol beside them or which state a birth date (rather than a baptism date), or which state a date as being "about".

The golden rule with the IGI is to treat it for what it is. It is an index. If you find an entry in the IGI, always look at the films/fiche of the original parish registers.
Used in this way, it is a great tool to help you with your family history research.