Electrical measurement apparatus comprising: a measurement arrangement configured to be disposed in relation to an electrical circuit which bears an electrical signal, the measurement arrangement being operative when so disposed to measure the electrical signal; a signal source operative to apply a reference input signal to the measurement arrangement whereby an output signal from the measurement arrangement comprises an electrical output signal corresponding to the electrical signal and a reference output signal corresponding to the reference input signal, the reference input signal being a square wave, a rectangular wave, a bit stream having sections of highs and lows with different duty cycles, a staircase waveform or a stepped waveform and being repeated over plural cycles; and processing apparatus which is operative to determine at least one of: a transfer function for the measurement arrangement; a change in a transfer function for the measurement arrangement; and the electrical signal, characterised in that the processing apparatus is further arranged to determine at least one cumulative representation, determination of the cumulative representation comprising summing plural received sections of the output signal, each of the plural received sections corresponding to at least a part and to a same part of the cycle of the reference input signal; and the at least one cumulative representation and the reference input signal are used to determine the at least one of the transfer function for the measurement arrangement, a change in the transfer function of the measurement arrangement and the electrical signal. a measurement arrangement configured to be disposed in relation to an electrical circuit which bears an electrical signal, the measurement arrangement being operative when so disposed to measure the electrical signal; a signal source operative to apply a reference input signal to the measurement arrangement whereby an output signal from the measurement arrangement comprises an electrical output signal corresponding to the electrical signal and a reference output signal corresponding to the reference input signal, the reference input signal being a square wave, a rectangular wave, a bit stream having sections of highs and lows with different duty cycles, a staircase waveform or a stepped waveform and being repeated over plural cycles; and processing apparatus which is operative to determine at least one of: a transfer function for the measurement arrangement; a change in a transfer function for the measurement arrangement; and the electrical signal, characterised in that the processing apparatus is further arranged to determine at least one cumulative representation, determination of the cumulative representation comprising summing plural received sections of the output signal, each of the plural received sections corresponding to at least a part and to a same part of the cycle of the reference input signal; and the at least one cumulative representation and the reference input signal are used to determine the at least one of the transfer function for the measurement arrangement, a change in the transfer function of the measurement arrangement and the electrical signal.