The buyer for a cell phone store wants to estimate the proportion of defective cell phones in a shipment of $10,000$ received cell phones. The shipment consists of $500$ boxes, each of which contain $20$ cell phones.
The buyer numbers the boxes $1$ through $500$, and using a random number generator, he selects ten numbers in that range (ignoring repeats). He then opens the boxes with the matching numbers and determines the proportion of cell phones that were defective.
What type of sample is this?