In reply to Robert Hillman.

Well, it is a fact that the tip_amount is zero for 40.26% of the 12.7 million observations in this data set!

However, you raise an excellent point. I agree that analysts should always question the accuracy of the data. I agree that when an analyst discovers a surprising result he should learn more about the data collection process (or report the issue to the data collection team). After thinking about your comments I investigated further. I found a "data dictionary" for the data which states that the tip_amount field "is automatically populated for credit card tips. Cash tips are not included." So that solves the mystery. You are correct that there is a systematic bias in the data: cash tips are not reported.

I have deleted the "suggested tweet" that you objected to. Thanks for encouraging me to dig deeper.