In the years since these pioneering studies, a number of modifications have been developed. These modifications relate to complication arising from violations of the statistical assumptions used in the early word and relate to adjustment in the design to accommodate more specific hypotheses. Useful papers which deal with the practical importance of many of the complication and adjustments are the work by Stephen Brown and Jerold Warner published in 1980 and 1985. The 1980 paper considers implementation issues for data sampled at a monthly interval and the 1985 paper deals with issues for daily data.