



 
Latoay, I think you are probably correct but it's sample variance not simple variance. It is however hard to know exactly what is going on in your teacher's mind. When you are taking measurements of some sort there may be different sources of variation that are measured by different variance measurements. Suppose you are a car maker, you have just produced a new model and you want to measure the gas mileage. You take two of the new cars and drive them the same route at the same speed and measure the gas mileage. Then you do it again with the same cars but a different route. Repeat a few more times over different routes and then from all the data calculate the mean gas mileage. There are two sources of variation here, one because the routes are different each time and the other because no two cars are identical:
I hope this helps,  


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