Regimental Strenths and Losses at Gettysburg, by Busey and Martin, calculates the strength of individual regiments principally from muster returns, but also from other sources, both going into that battle and at the end of it. Other primary source materials and reports provide a sense of attrition rates over the entire course of the battle. Some regiments are better reported/recorded than others, but it is possible to come up with reasonable estimates. Even so, refinements can still be made to the present day as more first-hand sources are uncovered in various repositories around the country, and that's for a battle on which more has been written about than any other from the war. My current mapping project tries to represent varying strengths of every participating regiment over the course of three days of fighting, a monumental although not impossible task. Absolute accuracy can never be achieved, but one tries to come reasonably close, and future historians can build upon what has been done before. Every other battle from the war could likewise be dissected in great detail, it just takes time, dedication, and a lot of effort. Thousands of letters, diaries, reminiscences, official reports, and unit histories all contribute to the end result. Some contain but a tiny piece of the puzzle, yet occasionally one finds the golden nugget.