Based on time series modelling, forecast the quarterly Retail Sales Index in 2019 and 2020.

Year/Quarter Retail Sales Index (Department Stores)

2016 1Q 103.8

2016 2Q 93.9

2016 3Q 93.2

2016 4Q 112.9

2017 1Q 100.3

2017 2Q 93

2017 3Q 94.6

2017 4Q 112.1

2018 1Q 102.7

2018 2Q 92.3

2018 3Q 93.6

2018 4Q 112.8