Network Analysis Question PERT/CPM?

Within the PERT you are sometimes provided with the optimisitc, pessimistic and likely values. If an activity on my critical path does not have these

Within the PERT you are sometimes provided with the optimisitc, pessimistic and likely values. If an activity on my critical path does not have these values do I assume that it has a variance of 1 or no variance at all?

or:Within the PERT you are sometimes provided with the optimisitc, pessimistic and likely values. If an activity on my critical path does not have these values do I assume that it has a variance of 1 or no variance at all?

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