6. A new strategy to ensure all patients living with HIV are treated with antiretrovirai therapy (ART) immediately after diagnosis was implemented in an area of Rwanda, in 2016 {Ross 2019). The effect of implementing this strategy on the proportion of patients initiating therapy within 30 days was analysed using an interrupted time series analysis. The time series plot is shown below. The grey line indicates the period when the strategy was implemented. u. . . . . . , . . . . . . ydgaun9adfuwf Ilot of proportion initiating ART within 30 days (y axis) over time. ' a) Describe the three predictor variables which would need to be tted to test the effect of the strategy for the time series shown above. (3 marks) b) In this study, there was no attempt to look for autooorrelation. Explain what is meant by autocorreiation and briey explain the plots that are used to look for it. (3 marks) c] In the gure, the researchers have shown the counterfactual as a dotted line. Explain this assumption of interrupted time series analysis and give one way we could test this assumption. (2 marks)