text: An antibiotic standard (shown in red) and test preparation (shown in blue) are applied at three dose levels to sensitive microorganisms on a layer of agar in petri dishes. The  stronger the  dose the larger the zone of inhibition of growth of the microorganisms. The biological response  is in this case the zone of inhibition and the  diameter of this  zone  can be used as the measurable response. The doses  are transformed to logarithms  and the method of  least squares is  used to fit two parallel lines to the data. The horizontal distance  between the two lines (shown in green) serves as an estimate of the  potency   of the test preparation relative to the standard.