We argue that heights of formation (HOFs) should not be assigned to spectral lines since a single line my sample very different layers of the atmosphere, depending on the physical parameter of interest and the technique employed to determine it. HOFs should be assigned to specific measurements. General expressions to compute these HOFs for measurements are derived. The equations are subsequently used to show, in representative solar measurements, the uncertainties produced by assigning HOFs to lines. Only weak lines can probe a single height of the atmosphere.