16.41 Newark College would like to include seasonality in its GPA forecasting model. The average GPA of the study body for each year was broken down into the average GPA by semester (Fall, Spring, Summer) and can be found in excel file grade inflation by semester.xlsx.

GPA	Semester	Year
2.74	Fall	2006
2.94	Spring	2006
3.20	Summer	2006
2.65	Fall	2007
2.69	Spring	2007
3.11	Summer	2007
2.61	Fall	2008
2.98	Spring	2008
3.05	Summer	2008
2.87	Fall	2009
2.83	Spring	2009
3.26	Summer	2009
2.60	Fall	2010
2.85	Spring	2010
3.01	Summer	2010
2.92	Fall	2011
2.89	Spring	2011
3.33	Summer	2011
2.83	Fall	2012
3.15	Spring	2012
3.12	Summer	2012
2.78	Fall	2013
3.06	Spring	2013
3.37	Summer	2013

a) Using multiplicative decomposition, forecast the average GPA for the student body for each semester during 2014.

c. Calculate the MAD for this forecast