Marcus Vance
Marcus writes the guides on this site: the breakdown of the percentage formula itself, the difference between percentage change and percentage difference, and the roundup of mistakes people make with percentages in spreadsheets and news coverage. Before writing full time he did a stretch of data reporting work where a chart labeled "percentage change" got confused with one labeled "percentage points" more often than you'd expect, sometimes changing what a quarterly number appeared to say. The guides here are built to head off that exact mix-up before it happens.
Across the Encore network his beat is everyday arithmetic, the kind of calculation people reach for a tool to double-check rather than a full course to learn.
Chris Terry
Chris built the calculator at the top of the homepage, the percent-change tool next to it, and the formula reference dataset underneath both. Before any guide of Marcus's ships, Chris runs its worked examples through the live tool and confirms the numbers line up.
You won't find him credited on a guide anywhere on this site. The editorial standards page is where he lays out how that checking process actually works.
If Something Looks Off
Both of them see whatever comes through the contact form. Chris takes anything about the calculator's behavior directly; a question about a specific guide usually gets forwarded to Marcus within a day or so.