Answer:
The difference between these types of samples has to do with the other part of the definition of a simple random sample. To be a simple random sample of size n, every group of size n must be equally likely of being formed. A systematic random sample relies on some sort of ordering to choose sample members.
Random sampling: If you choose all of your samples randomly, like sampling every 3rd person to walk into a room.
Systematic sampling: If you only sample certain people, like only blonds, or only guys. Stuff like that.
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