First of all we need data lots of it and good quality;
"lots of it" - I've found over five hundred footwear manufacturers and I'm sure there are a load more out there hiding. I've taken the sizing information from the larger and more influential firms first (still working on the others) and aim to get them all included in my database one day! It's a really time consuming process, far more than I thought.
"Good quality" - First off we need usable data, whilst some use can be made out of a size guide that for instance only compares EU and UK sizes, it's far better to have sizes that have a real relationship with the physical world by this I mean for instance EU sizes versus MM (or Inches) of actual foot length (and width though these are REALLY thin on the ground). Unfortunately we lose about 75% of the data by demanding this, though as mentioned the lost data can still serve us by offering votes in the debates over how sizes relate to each other. It's worth noting at this point that some companies provide for example UK, EU and US sizes against centimetre scales, these are interesting but dangerous too, interesting because Japan for example uses the centimetre scale for footwear but it's not really centimetres, it's close but not exact [nts: evidence] I emailed the questionable one and some firms responded saying that the centimetre scale was foot size, in which case it's almost certainly not Japanese size so at least one group is going to be unhappy (Japanese hoping to use the scale or people measuring their feet), anyhow if I was in any doubt even after the confirmation they didn't get included, that level of error is acceptable for sandals, wellies and slippers etc. but not for what we're trying to achieve.
So how does this data look, well showing the three major systems it maps out something like this (later versions will have more dots).
These are scatter diagrams showing a transparent grey dot where a manufacturer size maps to a foot length. Where the dot is darker there are more 'votes' from manufacturers for that particular mapping, so in a way black dots represent a consensus and this is generally a good thing.
So what does this show?
For these charts each category is given a colour they are Red for Women, Blue for Men, pale blue for Children, yellow for Toddlers and Green for Youth and Unisex. There are far fewer dots here because this is using our 'clean data', even information that had a size is excluded if that size cannot be confirmed as actual foot size. Looking at the US vs Inches chart it's clear that the anomaly previously is indeed due to the difference in representing size across the US adult sexes, looking at the other charts it's clear they are unisex with more purples (red and blue transparencies combining).
I'll continue to add more manufacturers to the database but the next step in the investigation is to generate some generalisations from the data and perhaps bust a few myths. US is always UK + 1, that sort of myth. [28th November 2014] Well the myth isn't quite busted see this for the continuation of the exciting journey into shoe sizing conventions.