Why the number of sessions is different when I change a funnel step from required to optional?

When you use a funnel report, you can set funnel steps as required or optional. Visitors who skip optional steps remain in a funnel and aren’t counted as drop-offs. Visitors who skip required steps are recorded as drop-offs.

But the session calculation in funnel steps can be tricky, and sometimes you may think that the session number is off. It’s not, and these examples will help you understand why.

Example 1

Comment:

In the second funnel definition, the algorithm matched the visitor’s path in the following way:

1. Found A at timestamp 01 (first A is taken)
2. Found B at timestamp 02 (first B is taken, ignoring A because it’s optional)
3. Found C at timestamp 03 (first C, after B, is taken)

• Found sequence: B, A, C
• Funnel definition: A, B, C
• Result: B, C

Example 2

Comment:
In the second funnel definition, the algorithm matched the visitor’s path in the following way:

1. Found A at timestamp 02 (A is taken)
2. Found B at timestamp 00 and 03 (last B is taken because it’s after A)
3. Found C at timestamp 01 and 04 (last C is taken because it’s after B)
4. Found D at timestamp 05 (D is taken)

• Found sequence: A, B, C, D
• Funnel definition: A, B, C, D
• Result: A, B, C, D

Example 3

Comment:
In the second funnel definition, the algorithm matched the visitor’s path in the following way:

1. Found A at timestamp 02 (A is taken)
2. Found B at timestamp 00 and 04 (last B is taken because its timestamp 04 is after A at timestamp 02)
3. Found C at timestamp 01, 03 and 05 (middle C is taken because its timestamp 03 is after A at timestamp 02)
4. Found D at timestamp 06 (D is taken)

• Found sequence: A, C, B, D
• Funnel definition: A, B, C, D
• Result: A, B, D