Whether we write fiction or nonfiction, we must conduct some degree of research, and it's important that the information we provide from that research is accurate. And in writing, accuracy equals credibility with our readers.
If our information is inaccurate, we risk alienating readers or possibly even losing them altogether because we lose credibility in their sight. We must also make clear the context of the information we provide so that readers do not misunderstand how we arrived at that information.
Numbers and statistics can be slippery things that affect accuracy.
For example, several different writers might mention the number of people aboard the Mayflower. One might cite the number of souls aboard when the ship left England. That number must be accurate. But another writer might cite the number aboard when they dropped anchor in the New World (excluding those who died en route but including those born during the voyage). Still others might cite the number who actually stepped ashore at Plymouth Rock or who were present at the first Thanksgiving meal. In each case, the number must be accurate, but the writer must also give the context for that number.
Another example is citations of the number of people killed during the War Between the States. Is the stated number combatants actually killed in action, or does it include those who died later as a result of wounds received? Or both? Is it only combatants, or does it also include civilians who were killed? The numbers given in various sources range from 600,000 to 750,000 or even more. Which is the accurate number? It depends on how one is calculating, and the writer must make that method clear to readers.
Dates can also become an issue.
For example, birth and death dates are sometimes tricky. The farther back in history one goes, the less certain these dates become because records are often missing or were inaccurately recorded, if they were recorded at all. Or we might inadvertently transpose numbers as we're taking notes during our research or write 1800 when we meant to write 1900. Similarly, when we are writing about when a structure was built (construction begun, completed, etc.?), renovated, or sold. Or when an organization was founded or actually started operation.
Locations and settings in our writing almost must be accurate.
Unless we're writing about some futuristic, out-of-this-universe fictional world, there's probably going to be a reader who has either lived in or visited the place we're describing or who has studied and knows it well. And that person will know if we get something wrong.
For example, if we write that the protagonist in our story raced down the interstate in West Knoxville to the I-75/I-81 split and went west on I-75, someone is going to recognize the multiple errors immediately, and we'll have lost all credibility with that reader. (Perhaps you know that the split west of Knoxville involves I-75/I-40, not I-81, and that I-75 is a north-south, not an east-west, interstate. The I-81 split is northeast of Knoxville and involves I-40, not I-75.)
One of my greatest disappointments with an author came when I was reading a historical novel by a well-known and prolific writer, and I found an egregious factual error. Perhaps other readers might not have caught it, but because American history happens to be my subject field, I did. (I'll not name the author out of respect for his well-deserved reputation, but the author's error will prove my point nonetheless.)
The author referred to an event in the story as having occurred in 1877, during the presidency of Theodore Roosevelt. I immediately did a double-take and reread the sentence to see if I had misread it. I had read it correctly, but the author was inaccurate in his writing. It was not Teddy Roosevelt but Rutherford B. Hayes who was president at the time referenced.
That author lost all credibility with me concerning historical matters. It wasn't a matter of opinion, interpretation, or perspective. It was clearly a factual error, something that might have been avoided had the author done the proper research to ensure accuracy. I still consider the author to be a good writer. I've heard him speak, respect the writing advice he gave, and even bought one of his books for my wife and had him autograph it for her. But I consider him a sloppy researcher.
Accuracy is the key to our credibility no matter what genre we're using as our medium. So do your research and get it right!