affected by lower sales volume of cabinets , the divestiture of our arrow and moores businesses , and an unfavorable sales mix of international plumbing products , which , in aggregate , decreased sales by approximately two percent compared to 2016 . net sales for 2016 were positively affected by increased sales volume of plumbing products , paints and other coating products and builders' hardware , which , in aggregate , increased sales by approximately five percent compared to 2015 . net sales for 2016 were also positively affected by favorable sales mix of cabinets and windows , and net selling price increases of north american windows and north american and international plumbing products , which , in aggregate , increased sales approximately one percent . net sales for 2016 were negatively affected by lower sales volume of cabinets and lower net selling prices of paints and other coating products , which , in aggregate , decreased sales by approximately two percent . net sales for 2015 were positively affected by increased sales volume of plumbing products , paints and other coating products , windows and builders' hardware . net sales for 2015 were also positively affected by net selling price increases of plumbing products , cabinets and windows , as well as sales mix of north american cabinets and windows . net sales for 2015 were negatively affected by lower sales volume of cabinets and lower net selling prices of paints and other coating products . our gross profit margins were 34.2 percent , 33.4 percent and 31.5 percent in 2017 , 2016 and 2015 , respectively . the 2017 and 2016 gross profit margins were positively impacted by increased sales volume , a more favorable relationship between net selling prices and commodity costs , and cost savings initiatives . 2016 gross profit margins were negatively impacted by an increase in warranty costs resulting from a change in our estimate of expected future warranty claim costs . selling , general and administrative expenses as a percent of sales were 18.9 percent in 2017 compared with 19.1 percent in 2016 and 18.7 percent in 2015 . selling , general and administrative expenses as a percent of sales in 2017 reflect increased sales and the effect of cost containment measures , partially offset by an increase in strategic growth investments , stock-based compensation , health insurance costs and trade show costs . selling , general and administrative expenses as a percent of sales in 2016 reflect strategic growth investments , erp system implementation costs and higher insurance costs . the following table reconciles reported operating profit to operating profit , as adjusted to exclude certain items , dollars in millions: .

2017: operating profit as reported: 1169
rationalization charges: 4
gain from sale of property and equipment: 2014
operating profit as adjusted: 1173
operating profit margins as reported: -15.3
operating profit margins as adjusted: -15.3
2016: operating profit as reported: 1053
rationalization charges: 22
gain from sale of property and equipment: 2014
operating profit as adjusted: 1075
operating profit margins as reported: -14.3
operating profit margins as adjusted: -14.6
2015: operating profit as reported: 914
rationalization charges: 18
gain from sale of property and equipment: -5
operating profit as adjusted: 927
operating profit margins as reported: -12.8
operating profit margins as adjusted: -13

operating profit margins in 2017 and 2016 were positively affected by increased sales volume , cost savings initiatives , and a more favorable relationship between net selling prices and commodity costs . operating profit margin in 2017 was negatively impacted by an increase in strategic growth investments and certain other expenses , including stock-based compensation , health insurance costs , trade show costs and increased head count . operating profit margin in 2016 was negatively impacted by an increase in warranty costs by a business in our windows and other specialty products segment and an increase in strategic growth investments , as well as erp system implementation costs and higher insurance costs . .......................................................... . .................................................................. . ..................................... . ........................................................ . ............................................ . ............................................. .

Questions:
what were gross profit margins in 2017?
and in 2016?
so what was the difference between these two values?
and the percentage change during this time?

Answers:
34.2
33.4
0.8
2.4%