update lesson

_episodes/03-HLT_efficiency.md

@@ -12,7 +12,27 @@ objectives:
 > Set up your machine following instructions in [setup][lesson-setup] first.
 {: .prereq}
 
-The goal of the following exercises is to learn how to access and play with the trigger objects in our data and compute the efficiency of a specific HLT path, having a look also at its Level 1 (L1) seed. The focus will be on a HLT path used during the 2016 data-taking to select events with a certain amount of missing transverse energy: `HLT_PFMET170_HBHECleaned_v*`.
+> ## Objective
+> The goal of the following exercises is to learn how to access and play with the trigger objects in our data and compute the efficiency of a specific HLT path and look also at its Level 1 (L1) seed. 
+>
+> The focus will be on a HLT path used during the 2016 data-taking to select events with a certain amount of missing transverse energy: `HLT_PFMET170_HBHECleaned_v*`.
+{: .objectives}
+
+## Ex 1: Access trigger bits and compute a MET trigger efficiency
+
+We will first run this exercise on MiniAOD format, then run it again on NanoAOD.
+
+> ## MiniAOD
+> The MINIAOD format was introduced at the beginning of Run 2 to reduce the information and file size from the AOD file format. 
+> For Run 2 analyses, most of the analysis groups at CMS skimmed the centrally produced MiniAOD files into smaller, analysis-specific ROOT Ntuples. 
+> This means that several redundant versions of Ntuples for different analysis groups are stored in the limited CMS storage spaces.
+>
+>{: .solution}
+
+> ## NanoAOD
+> A centrally maintained NanoAOD format was proposed in 2018, aiming for a common Ntuple format that can be used by most of the CMS analysis groups. 
+> Information about the NanoAOD format can be found [here](https://cms-nanoaod-integration.web.cern.ch/integration/master-102X/mc102X_doc.html#HLT). 
+>{: .solution}
 
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