This chapter presents a case study where lost sales have to be determined during the period of interruption. It highlights an alternative approach to model lost sales in time series analysis with a seasonal adjustment. Under this approach, optimized seasonal indexes, monthly time series model and composite model are created. It is clear that quarterly time series model offers less variability than monthly time series model. Hence, quarterly series model is considered to be the best choice for estimation of lost sales. This approach takes care of seasonality trends and adjusts them into Optimized Seasonal Indexes. This makes the estimation process further easy as data taken for this is less variable modeled on quarterly series.