Design of the Month

It was considered necessary to attempt further to measure the influence, if any, on bleb size of the position of the injection on the back of the animal and of the order (in time) of injection, as well as to estimate the contribution made by animal variation to the error of the method. ... It was decided to carry out an experiment with six rabbits and six positions for injection on the back of each rabbit and to base the experiment on a Latin square design.

A 6 x 6 Latin square was therefore selected by the procedure described by Fisher & Yates [1938]. ... Arabic numerals (1 to 6) refer to the animals, letters (a to f) to the position on the backs of the animals and Roman numerals (i to vi) to the order of injection. Thus each animal was injected once and once only in each position and each position was the rth to be injected (r=i, ii, ..., vi) in only one animal. Also, the sites (a to f) were so arranged that three of them (a, c and e) lay near the animals' vertebrae, while the other three (b, d and f) occupied more lateral positions. In Table 1 are also shown the actual bleb sizes (in square centimetres) 20 min. after the injection.