An apparatus for determining a signal quality of an electrocardiography signal derived from at least one sensor connected to a patient comprising: a detector configured to receive the electrocardiography signal and determine electrocardiography data representing electrocardiography signal for use in determining at least one patient parameter; a measurement processor electrically coupled to the detector, the measurement processor configured to determine a first signal quality value by identifying at least one feature of the electrocardiography data and by comparing the at least one feature of a first set of the determined electrocardiography data with the at least one feature of a second set of the electrocardiography data to determine a feature variability value and using the feature variability value to determine a stability value representative of the quality of the electrocardiography signal, characterized in that said measurement processor is configured to measure a second signal quality value associated with the electrocardiography signal, the second signal quality value being measured prior to the electrocardiography signal being processed by the detector, wherein the second signal quality value is determined by evaluating a randomness of a finite sequence of data samples included in the electrocardiography signal to determine a complexity value associated with the electrocardiography signal and comparing the complexity value with a threshold to determine an amount of noise in the electrocardiography signal, and said measurement processor is configured to evaluate the randomness of the electrocardiography signal by converting the electrocardiography signal into a binary signal, sequentially scanning the binary signal, incrementing a complexity counter by one upon detecting a change in a subsequence of consecutive characters in the binary signal and comparing a value of complexity counter with a threshold to determine the complexity value associated with the electrocardiography signal. a detector configured to receive the electrocardiography signal and determine electrocardiography data representing electrocardiography signal for use in determining at least one patient parameter; a measurement processor electrically coupled to the detector, the measurement processor configured to determine a first signal quality value by identifying at least one feature of the electrocardiography data and by comparing the at least one feature of a first set of the determined electrocardiography data with the at least one feature of a second set of the electrocardiography data to determine a feature variability value and using the feature variability value to determine a stability value representative of the quality of the electrocardiography signal, characterized in that said measurement processor is configured to measure a second signal quality value associated with the electrocardiography signal, the second signal quality value being measured prior to the electrocardiography signal being processed by the detector, wherein the second signal quality value is determined by evaluating a randomness of a finite sequence of data samples included in the electrocardiography signal to determine a complexity value associated with the electrocardiography signal and comparing the complexity value with a threshold to determine an amount of noise in the electrocardiography signal, and said measurement processor is configured to evaluate the randomness of the electrocardiography signal by converting the electrocardiography signal into a binary signal, sequentially scanning the binary signal, incrementing a complexity counter by one upon detecting a change in a subsequence of consecutive characters in the binary signal and comparing a value of complexity counter with a threshold to determine the complexity value associated with the electrocardiography signal.