It is an outcome from over 400 data points. We're expecting to see events outside a 3 sigma boundary. In fact, 3 sigma is basically the 1:400 boundary. Strictly it is 1 in 370 [0] for anyone who takes Wikipedia at face value.

In the study they seem put the P value of the taxi driver outcome at <0.01 (in Table 2, although I admit I haven't read the thing especially closely). For 400 data points, we expect there to be around 4 values with a P value less than <0.01.

[0] https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rul...