Table 2.

Qualitative comparison of algorithms.

Method and obfuscation		Approximation error	Runtime wait	Risk:Hub	Risk:Hub+Site
Count
	None	Large	Very small	Medium (–)	Medium (–)
	Mask	Large	Very small	Zero	Zero
	MPCa	No changeb	Medium	Zero	Zero
HLL7c
	None	Medium	Small	Medium (+)	Medium (+)
	Shuffle	No change	No change	Small (+)	No change
	Rehash	No change	Medium (+)	Zero	No change
	Mask	Medium (+)	Medium (–)	Zero	Zero
	MPC	No change	Large	Small (+)	Small (+)
	Shuffle+MPC	No change	HLL7+MPC	Very small (+)	HLL7+MPC
HLL15
	None	Small	Medium	Large (+)	Large (+)
	Shuffle	No change	No change	Small (+)	No change
	Rehash	No change	Medium (+)	Zero	No change
	Mask	Large (+)	Medium (–)	Zero	Zero
HashedIDs
	None	Zero	Medium (+)	Very large (+)	Very large (+)
	Rehash	No change	No change	Zero	No change

^aMPC: multiparty computation.

^bNo change: the value is the same as the method without any obfuscation.

^cHLL: HyperLogLog.