You're trying to improve patient satisfaction scores in your hospital's emergency department. You decide to poll 40 patients a day for three weeks. Your charge nurse says that the quality of service really fluctuates depending on the time of day. So your daily sample consists of 10 patients polled at 8 AM, another 10 at noon, another 10 at 5 PM, and the final 10 at 9 PM. This is an example of what kind of sampling?