the challenge here is to identify the factors in each level that actually affect the thing you're trying to predict if ' trying to predict overall sat scoresfor examplei know that depends in part on the individual child that' taking the testbut what is it about the child that matterswellit might be the geneticsit might be their individual healththe individual brain size that they have you can think of any number of factors that affect the individual that might affect their sat score and then if you go up another levellook at their home environmentlook at their family what is it about their families that might affect their sat scoreshow much education were they able to offerare the parents able to actually tutor the children in the topics that are on the satthese are all factors at that second level that might be important what about their neighborhoodthe crime rate of the neighborhood might be important the facilities they have for teenagers and keeping them off the streetsthings like that the point is you want to keep looking at these higher levelsbut at each level identify the factors that impact the thing you're trying to predict can keep going up to the quality of the teachers in their schoolthe funding of the school districtthe education policies at the state level you can see there are different factors at different levels that all feed into this thing you're trying to predictand some of these factors might exist at more than one level crime ratefor exampleexists at the local and state levels you need to figure out how those all interplay with each other as well when you're doing multi-level modeling as you can imaginethis gets very hard and very complicated very quickly it is really way beyond the scope of this bookor any introductory book in data science this is hard stuff there are entire thick books about ityou could do an entire book about it that would be very advanced topic so why am even mentioning multi-level modelsit is because 've seen it mentioned on job descriptionsin couple of casesas something that they want you to know about in couple of cases 've never had to use it in practicebut think the important thing from the standpoint of getting career in data science is that you at least are familiar with the conceptand you know what it means and some of the challenges involved in creating multi-level model hope 've given you those concepts with thatwe can move on to the next section