The Error of the Red Cell Count.

The staffs of haematological laboratories spend much of their time counting red cells. The precision of this estimation is therefore of practical importance, and many experiments have been designed to establish the error of the technique. In spite of this, the most widely divergent views are still held as to the reliability of single observations. In 1881 Lyon and Thoma showed that the 'standard error of counts made on the same sample of blood was roughly proportional to the square root of the number of cells counted. Thus in a count of 100 cells the standard error would be roughly ./ 100, and repeat counts on the same sample might by chance vary from 80 to 120 cells, or, in counts of 500 cells, the standard error would be approximately V/ 500 or 4.5 per cent. In 1906-7