Multilevel modelling and time series analysis in ... - ERSO - Swov

Chapter 2These results are intuitive (i.e. an increase in speeding and drinking-and-drivingcontrols results in a reduction of road accidents). In the next section, we will seehow this effect may vary when adding more structure to the data.2.3.4.2. A two-level Poisson modelWe will now fit a two-level model, in order to investigate the regional variation ofthe effect of enforcement on the number of road accidents. We shall start withthe random intercept model:▪ Remove the terms alcohol and speed from the model.▪ Click on accs in the Equations window, select j-2 from the N levels: dropdown list and region from the level 2(j) drop down list, and click Done.▪ Click on the variable cons in the Equations window and set cons to berandom at the j(region) level.As there are only 12 regions at the higher level, it is recommended to use theRIGLS estimation method, which provides less biased estimates of the variancethan the IGLS when there is limited number of higher level units.▪ Select RIGLS from the Estimation menu of the main window▪ Click the Start button on the toolbar of the main window to run the model.The results are as follows: