To evaluate the real chronic effects of China 's long - term air pollution control plan , it is necessary to exclude COVID-19 effects in 2020 . One possible solution is to predict the air pollutant concentration in each city in 2020 according to annual data in the past several years as a normal condition and then compare the difference between the data and real annual data in 2020 . Considering the advantages of grey prediction theory in small sample prediction ( Wu et al . , 2013a(Wu et al . , , 2013b , it has been used to predict air quality indicators with limited original data and few samples ( Zhou et al . , 2020b;Wu et al . , 2018;Chen and Pai , 2015 ) . In this paper , the GM(1,1 ) model can be described as follows :