I have some differences on the certificates and census information.

Birth Certificate:

Name: Edith Nellie Hardiman

Birth Date: 16 Sep 1892

Birth Location: Compton Chamberlayne, Wiltshire, England

1901 Census

I do not have access to this census entry but I think it will be her:

It seems to indicate 1893 there but I assume it could theoretically calculate to 1892. Depends what the image states.

1911 Census

Name: Edith Ellen Hardiman

Birth Date: 17 Years Old

Birth Location: Compton, Wiltshire, England

• 
  Nellie could be considered Ellen I assume?


• 
  Compton could be considered Compton Chamberlayne I assume?


• Being 17 indicates a birth year of approx. 1894.

Marriage Certificate 1

She married my Great Grandfather in 1912 and it states:

Name: Edith Ellen Hardiman

Birth Date: 20 Years Old

So this calculates to 1892 again.

Marriage Certificate 2

My Great Grandfather died unexpectedly and thus Edith remarries in 1931:

Name: Edith Ellen Truckle

Birth Date: 37 Years Old

• 
  Truckle is correct and this was her first married surname.


• Being 37 indicates a birth year of approx. 1894.

1939 Register

Name: Edith E Smith

Birth Date: 24 Sep 1895

• Smith is correct as that is her new married surname.


• 
  1895 is yet another year!

1939 Register Addendum

Here are all entries on the register for the household:

This is my tree:

Although my father has dementia now he over the years he told me that my Grandfather (Frederick George) had three brothers. Note that all of them have passed away thus I can present this information.

Death Certificate

She died in 1977:

Name: Edith Nellie Smith

Birth Date: 24 Sep 1894

Birth Location: Compton Chamberlayne, Wiltshire, England

So we have two names:

• Edith Nellie Hardiman


• Edith Ellen Hardiman

And we have a few birth entries:

• 16 Sep 1892


• 1893 (maybe - 1901 census - could be 1892)


• 24 Sep 1894


• 24 Sep 1895

Since I have the actual certificates I can confirm that her father remains the same Samuel Hardiman throughout.

At the moment I have used the latter date of 1894 since this is what they all thought:

My gut tells me to just ignore the 1939 Register. They must have been confused. But then my gut also tells me that I should really use the 1892 date as the preferred since it is when she was born:

So my question is two-fold really:

1. Is Nellie / Ellen interchangeable?


2. Which birth date should be preferred? Birth or death?

Part of your difficulty lies in the way the big data sites like Ancestry encourage us to 'do genealogy' -- we look for our 'people' by cherry-picking the most likely matches to the person we're seeking, then we try to assemble all the bits we've found into a recognizable portrait of a person.

Hint-based or index-driven searching encourages us to take the records we find out of context. We lose valuable clues that way. We can better make use of the information if we understand the sources are looking at -- which agency created them, and for what purpose -- and if we evaluate the information in each source after we have considered the source itself.

Elizabeth Shown Mills' QuickLesson 17: The Evidence Analysis Process Map gives us one method of evaluating sources and the information inside them.

• 
  Sources are the documents and other items you've found. They are containers for information.


• 
  Information is what we learn from each source.


• Information becomes Evidence when we use it to answer a specific research question such as When was Edith Nellie Hardiman born?
  

To answer research questions, we can use the Genealogical Proof Standard:

To reach a sound conclusion, we need to meet all five components of
the GPS.

1. Reasonably exhaustive research.


2. Complete and accurate source citations.


3. Thorough analysis and correlation.


4. Resolution of conflicting evidence.


5. Soundly written conclusion based on the strongest evidence.

When building a case, keep in mind what Dr. Thomas W. Jones says about the Perils of Source Snobbery. It's easy to assume that primary sources which have direct evidence about our research question are the best sources and call it a day, but those can be wrong.

Statements such as this are a red flag:

My gut tells me to just ignore the 1939 Register. They must have been confused.

Have you studied how the 1939 Register was created? Do you understand its purpose? Do you know who the informant is for the information contained within?

We tend to assume that all documents have information reported by the person whose information it is, but in actuality, we don't always know who the informant was. You say "They must have been confused." -- who do you mean by "they"?

Let's assume that your identification is correct and this is the right entry for your person. Now consider -- do any of us know what our birth date is? We were there, to be sure, but we weren't capable of reading a clock or a calendar! We only know our own birth date or age because someone else told us -- it is secondary information. It is also more distant in time from the birth than the birth certificate is (to pick the most extreme example). And might it be possible that someone else gave the information, or that the 1939 Register entry belongs to a different person? A good proof statement accounts for all of these things.

If you can't make an explicit statement about what your 'gut' is telling you, then it may be that you aren't confident yet because:

• You haven't made a reasonably exhaustive search. Adding a baptism record, school records, a family notice, and considering the larger context, such as her place in the sibling set, can help you make a stronger case. Don't discount indirect evidence -- where you combine information from several sources. In some instances you can make a strong case with the indirect evidence even when direct evidence is lacking.


• You don't understand your sources well enough.


• You haven't analyzed your sources enough.

Attempting to write a proof statement can help you find your hidden assumptions and the 'holes' in your research. Evidentia is a program that can walk you through the process of extracting the claims from a source, analyzing the information inside, and writing a proof statement about what you've found (or recording that you still aren't sure yet). Another aid for evidence analysis is the online application Lineascope.

** A note on Documentation:**

Even though Ancestry allows us to have variant birth events in our online trees and mark one as preferred, recent changes to the tree system make that unwieldy. If you Show the Family Events in a person's profile, Ancestry now shows you all of the different birth and death events, not just the preferred one. In a recent 'Barefoot Genealogist' video, Tidying Up Your Genealogy, Crista Cowan suggests that best practice is to have only one birth and death event, rather than have multiples to keep track of variants. (It may be that it is better for Ancestry if we clean up our trees, but that discussion is outside the scope of this question and answer.)

So if you want to keep track of variant information, what are ways you can do so?

• In the Notes on our tree (but a caution: since the recent introduction of MyTreeTags™, the auto-save on Notes has become unreliable for some users)


• In the description of the (single) birth or death event on our online tree


• In a separate file (your research report you've written to yourself) on your desktop computer, and/or in your genealogy software


• In programs like Evernote or OneNote, Evidentia, Lineascope, etc.


• In a research journal, using a program like Scrivener, Word, RedNotebook Portable etc.


• In an old-school paper notebook, such as a bullet journal.

You've made a good start by writing up the question for this site -- now take advantage of that and look for clues in what you already have, plus be on the lookout for more sources that might give you more confidence in your conclusion.

Resources:

• 
  England Record Finder at the FamilySearch Wiki


• 
  Strategic Research Logs -- England at the FamilySearch Wiki


• GenGuide - UK and Irish Genealogy Sources, Aids and Contacts


• 
  1939 Register Research Guide at TNA


• 
  Inside the 1939 Register guide from Lost Cousins