15% for median service. 10% for 1 or more standard deviations below, and 20% for 1 or more standard deviations above. So you pay 15% on 2/3 of your meals, 10%

15% for median service. 10% for 1 or more standard deviations below, and 20% for 1 or more standard deviations above. So you pay 15% on 2/3 of your meals, 10% on 1/6, and 20% on 1/6. But the mean is also 15%. Giving 20% to everybody is Lake Wobegon syndrome, where all the servers are above average. If you don't adjust your tip based on the quality of service, you remove the incentive to strive for above-average service. You end up with servers that sit down at your table when taking your order and chitchat with you, instead of just doing their job professionally while you can see or hear them, and bitching about it in private later.