Editing (Symmetric) Private Information Retrieval (section)

===Cost parameters===
The most common cost parameter used to characterise a given (Q)(S)PIR protocol is:
*'''Communication complexity''': total number of (qu)bits exchanged between the user and the server(s) throughout the protocol.
For (Q)(S)PIR protocols in general:
*'''(Q)(S)PIR capacity''': maximal achievable ratio of the retrieved database element size to the total download size.
Some less common cost parameters include:
*'''Storage overhead''' (for multi-database (Q)(S)PIR protocols): ratio between the total number of (qu)bits stored on all servers and the number of (qu)bits in the (resp. quantum) classical database. 
*'''Access complexity''': total amount of data to be accessed by the server(s) for answering queries throughout a (Q)(S)PIR protocol.