A method of analysing gas chromatography data is described. During the method, a first response factor data set acquired from a gas chromatograph (GC) apparatus during a procedure on a calibration or reference gas sample at a first time is received. One or more additional response factor data sets acquired from the gas chromatograph apparatus during a procedure on a calibration or reference gas sample from one or more later times are received. The method comprises calculating a measure of uncertainty for at least one compound of the reference gas sample from the first and additional response factor data sets. The one or more later times are during an operational period of the gas chromatograph apparatus. The measure of uncertainty may be used to, for example, identify the necessity to perform a maintenance action in the GC or to assess whether the GC is in a healthy or unhealthy condition.