Our findings indicate that doctors' decision-making about slow labour is influenced by the way in which information is presented graphically. Doctors are less likely to intervene during labour if the cervical dilation is drawn on a partogram with a low x to y ratio so that the progress line has a steep gradient than if the x to y ratio is higher so that the line is flatter. Moreover, if the latent phase of labour is drawn on the partogram labour seems longer with slow progress, and doctors are again more likely to intervene than if the information is omitted from the graph. Since the rate of progress is the same in both cases, the difference in decision-making must be due to a change in perception.