The Occupant Response Model is a sub-model of the Risk-Cost Assessment Model for apartment building being developed at the National Fire Laboratory of the National Research Council of Canada. The Occupant Response Model simulates human response to information indicating the presence of a fire. The perception of the information, interpretation and resulting action is called the PIA process. The PIA process is performed at each state of fire growth obtained from the Fire Growth Model. Interpretation levels have been defined according to the information sources reaching the occupants in the building. Further, the interpretation level introduces a time delay to interpret information and to take actions before the occupants start evacuation. The probabilities of perception of the various types of information depend on the state of fire growth, the occupant location and the actions of other occupants. Calculations are made from the nearest location to the fire to the furthest for three interpretation levels. Each interpretation level determines the delay time before the occupants start to evacuate the building. The model outputs are the probabilities of occupants starting to evacuate at six timeframes before flashover. These values will be used by the Occupant Evacuation Model and the Fire Brigade Action Model, which are parts of the Risk-Cost Assessment Model.