Pongkitwitoon, M., and W. Klongdee. “Using GM(1,1) with sample standard deviation to forecast downtrend rainfall for small sample in Khon Kaen, Thailand”. Engineering and Applied Science Research, vol. 43, Nov. 2016, pp. 111-3, https://ph01.tci-thaijo.org/index.php/easr/article/view/69709.