1 Introduction  
2 Methods for Factual Probing  
2.1 Probing Inputs  
2.2 Probed PLMs  
2.3 Outputs  
3 Datasets for Factual Probing  
4 Insights about Knowledge Retention and Prompt Optimization  
4.1 Factors Affecting Knowledge Retention  
4.2 Should Prompts be Optimized?  
5 Obstacles to Adopting PLMs as KBs  
6 Related Work  
7 Discussion and Future Work