The graph of the relation between @[email protected] and the proportion of individuals responding with an all-or-none effect; it is essentially the graph of the @[email protected] of an occurrence (or the proportion of a population exhibiting an effect) against @[email protected] Typical examples of such all-or-none effects are mortality or the incidence of cancer. The @[email protected] curve is the graph of the relation between @[email protected] and the magnitude of the biological change produced measured in appropriate units. It applies to measurable changes giving a graded response to increasing doses of a @[email protected] or @[email protected] It represents the effect on an individual animal or person, when biological variation is taken into account. An example is the increased effect of lead on the @[email protected] synthesis, e.g., on activity of the enzyme 6-amino laevulinic acid dehydratase in blood @[email protected] or coproporphyrin levels in urine.