icity*
   - e.g., no one who who go to the library if not encouraged but who won't go to the library if encouraged
 
 3. This is a classic design trumps analysis problem


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## Missing Data

Problems:

 - Missing data is a threat to representativeness
 
 - Missing data increases our uncertainty
 
Solutions:

 - Case deletion
 
 - Imputation
 
???

Case deletion: per analysis or entirely

Imputation: single or multiple

Sensitivity analysis


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## Cluster random assignment

 - Cluster randomization is fi