The following table represents data for emergency department visits. NOTE:. indicates missing value

Year	Q1	Q2	Q3	Q4
2014	.	.	1,613	1,262
2015	1,311	1,235	944	879
2016	1,385	996	1,516	975
2017	1,276	1,063	1,318	1,087
2018	1,242	1,102	1,536	831
2019	1,122	1,059	.	.

If the data were entered into MS Excel as shown below using dummy variables to identify the quarter in which the visits occurred:

Year	Visits	Q1	Q2	Q3
2014	1,613	0	0	1
2014	1,262	0	0	0
2015	1,311	1	0	0
2015	1,235	0	1	0
2015	944	0	0	1
2015	879	0	0	0
2016	1,385	1	0	0
2016	996	0	1	0
2016	1,516	0	0	1
2016	975	0	0	0
2017	1,276	1	0	0
2017	1,063	0	1	0
2017	1,318	0	0	1
2017	1,087	0	0	0
2018	1,242	1	0	0
2018	1,102	0	1	0
2018	1,536	0	0	1
2018	831	0	0	0
2019	1,122	1	0	0
2019	1,059	0	1	0

Which quarter represents the base, or reference category?

a.Q2

b.Q4