The Lee–Carter model is a numerical algorithm used in mortality forecasting and life expectancy forecasting. The input to the model is a matrix of age specific mortality rates ordered monotonically by time, usually with ages in columns and years in rows. The output is a forecasted matrix of mortality rates in the same format as the input. The model uses singular value decomposition (SVD) to find: Many researchers adjust the vector by fitting it to empirical life expectancies for each year, using the and generated with SVD. When adjusted using this approach, changes to are usually small.