Explainer
Why two bookmakers’ margins usually cannot be compared
A margin is a number about a sample. Change the sample and the number changes, often by more than the gap between two books.
Every margin published on this site rests on a set of prices somebody read on a particular day from a particular place. The size of that set runs from two prices to eight hundred and sixty. Two of the eight came from somewhere other than the bookmaker’s own current line. One is three years old.
None of that is a criticism of the research. It is what happens when several bookmakers geo-block their own prices, render them in a way that shows nothing to anybody outside the country, or serve a demonstration coupon full of matches that are not being played. The dishonest response would be to print all eight figures to two decimals in a single column and let the reader assume they answer the same question.
So the board grades each margin on three things that are all properties of the sample rather than of the book: how many prices it rests on, whether they came from the bookmaker itself, and whether they span more than the top flight. Source is tested first, because a figure that did not come from the bookmaker is not rescued by a large sample.
The practical consequence is that comparisons should be made inside the top grade and treated as indicative outside it. A book measured on eight of its own prices may genuinely be the sharpest in the market, and eight prices cannot establish it.
Questions this page gets asked
Why not just wait until every book is measured the same way?
Because that day may not arrive, and a blank column is not more honest than a graded one. Publishing the figure with what it rests on lets a reader discount it correctly; publishing nothing lets them go and read a site that prints all eight without a caveat.
Does an old measurement still tell you anything?
Something, and less than a current one. Margins move. A three-year-old capture establishes roughly where a book positioned itself then, which is worth more than nothing and much less than a reading from this month.