A procedure for testing predictive validity with longitudinal data. Use N-1 observations to calibrate the forecasting model, then make a forecast for the remaining observation. Replace that observation and draw a new observation. Repeat the process until predictions have been made for all observations.  Thus, with a sample of 57 observations, you can make an out-of-sample forecast for each of the 57 observations. This procedure is also called N-way cross validation.