If a witness were testifying in court and began to relate a conversation they had outside of the court, the attorney for the opposing party would immediately object to the testimony on the basis of the hearsay rule of evidence. Almost uniformly in the U.S. court system testimony about what someone said, with a few specific exceptions, if inadmissible and objectionable. If the witness manages to say something about their conversation before the attorney can object, the Judge can order the testimony stricken from the court record. In the genealogy world, we not only accept hearsay, but many people think that hearsay is real evidence.
Let me give a some examples. Let's suppose we examine the U.S. Census. A census worker (enumerator) visits the houses in his or her district asking a list of questions. The person responding relates the answers from memory, often giving information the person heard from others (second hand). The census worker then writes the information down on the form and turns it in to the supervisor. Eventually, the information is tabulated and the results published by the U.S. Census Bureau. Some years later, other workers examine the census sheets and make a list of the names (an index). The index is now, at least, two or three steps away from the source of the original information. Could the index be used in court? Well, possibly, there is an exception for various kinds of records, such as government records and records compiled in the course of a business. But depending on the circumstances, the trier of fact (a jury for example) may be instructed to consider the reliability of the information before using it to make a decision.
Another example, suppose the government requires people to fill out a form, such as the World War I Draft Registration forms. Years later, other people look at the forms and make a list of the names and some of the information on the forms. The list is then alphabetized and some years later put into a computer. How reliable is the index and would it be considered hearsay in a court? Yes, by any definition the index is hearsay. The index might be admissible in a court, depending on the circumstances, but again, likely there would be qualifications about its reliability.
Unfortunately, there is no judge or attorney standing next to genealogical researchers telling them that the information they are looking at is hearsay and unreliable. Many of the large compilations of genealogical information indiscriminately mix digitized images of original documents with lists or indexes without qualifying either record. Unsophisticated researchers will look at the index of the records and assume that whatever they find there is "correct" and dutifully copy down the information without qualifying it at all. Many researchers confuse the indexes with the actual records and because the index is printed and more readable, it is for that reason, more reliable.
A couple of days ago, I sat with a researcher who was looking for information about the death of a relative. He first asked if he could find an obituary. Since the relative lived in Texas, we suggested the Texas death index as a starting point for research. We found the relative's name in about five minutes (the computers were slow). The reaction of the researcher was interesting. For him, the search was over. Since he found the name listed in an index, that is all he needed. I suggested he may wish to look at the death certificate or continue to look for an obituary, but he was now on to the next level, having accepted the index record as the truth.
Indexes are not primary source information. They are often wrong, as anyone who has worked with the U.S. Census records will know. But seeing information on a computer screen or in print does strange things to most people. There is an immediate assumption that the task of searching is over, even though if the evidence were being presented in court, an attorney would have immediately objected to the reliability of the evidence.
Finding a name in an index is just the beginning of the research process, not the end. In every case, where possible, the original record should be examined. In the case of census records, the original record is the census form. In some cases, the original record is the list, such as tax rolls or enlistment records, but any time you are looking at what someone copied from the original record there is a possibility of error. It is the possibility of error, not just the error itself, that makes for the hearsay rule in court. It should also be the underlying rule for genealogical research, always look to the original source of the information, as much as possible.
The Genealogical Proof Standard consists of five elements. The first element requires a reasonably exhaustive search. Relying solely on an index violates this first element meriting confidence in the reliability of the proof. Looking only at an index, even if that index is published in a major online resource, it does not provide credible evidence, merely a suggestion that credible evidence may exist. An index should always lead to other, more reliable, records.
I realize that it is sometimes discouraging for a novice researcher, or any researcher, for that matter, to realize that having found the ancestor's birth date in a list, they still have to keep searching. But as in court, genealogy can only move forward with reliable information when a reasonably exhaustive search is made and it is always reasonable to seek out the original source and not rely solely on the index.