This is the Bristol Stool Chart, an actual medical aid to classify human faeces. It was devised by a pair of researchers at the Bristol Royal Infirmary and published in the Scandinavian Journal of Gastroenterology in 1997. According to the methods section, 66 volunteers kept a diary of their stool form (on a 7-point scale), the frequency, and the stools were weighed (wet weight, I assume). Their eternal contribution to the edifice of human knowledge is summed up in this magisterial chart.
The scale goes from constipated to brown trouser time and Types 3 and 4 are "ideal". Of course we could have all told them that a priori, rather than having to reach the same conclusion a posteriori (snort), but theres nothing like nailing down an assumption empirically.
And now if only some kind Japanese readers could translate this chart.