Analyze the following statements about testing user referral programs for growth, and identify which are accurate:
- Statement I: Measuring the virality coefficient (k factor) in a referral program is unnecessary if the initial user response is positive.
- Statement II: Testing various incentive structures in a referral program is crucial to determine which one maximizes user participation and referrals.
- Statement III: A referral program's success can be solely judged by the number of new users it attracts, without considering user engagement or retention.
Select the statement(s) that correctly reflect principles of effectively testing user referral programs for growth.