In using the "rule of seven", a quality control approach, it is generally accepted that a process is out of control when in a production run seven or more samples lay on one side of the mean on the control chart. what is the probability that a run of seven samples, appearing on either side of the process mean is due to random variable?

1. 0.16%
2. 0.78%
3. 1.56%
4. 1.99%

correct answer is 3. can somebody please explain how we get this.

Thanks.