Systematic sampling is where you basically just set a rule on how to select each person. In this case, the rule would be to select every 5th person that drives through the toll.
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Extra info:
Stratified random sampling is the idea of breaking people into different mutually exclusive homogenous groups (called strata). An example would be to have a group of all boys and another group of all girls.
Voluntary response sampling is exactly what it sounds like: it's whenever you have people who volunteer their responses. In other words, only those who choose to be in the sample will counted. This is more than often a very biased form of sampling.
Simple random sampling (SRS) would involve things like using a random number generator, picking names out of a hat, rolling a dice, etc etc.
