To find the average grades scored by students in science and math, the principal of a school sends a questionnaire to the top five students in each grade of the high school.
Is the sample used by the principal random? Will a generalization from this sample be valid?

The sample used by the principal is (random, not random).
A generalization made from this sample is (valid, not valid).

Question

To find the average grades scored by students in science and math, the principal of a school sends a questionnaire to the top five students in each grade of the high school.
 Is the sample used by the principal random? Will a generalization from this sample be valid?

The sample used by the principal is  (random, not random).
A generalization made from this sample is  (valid, not valid).

AZ · Answer

Hi!

The principle is trying to find the average grades scored by students.
He's only asking the top five students in each grade.

If you ask only the top five students of each grade, will you be able to find the average grade scored by all students?

Is he asking students randomly or non random?

Finally, if he's only asking the best students, does that mean we can use the results that the professor gets and say that it's the average grade by all students?
So basically, would the generalization be valid or not valid?

AZ · Answer

as an example-
Let's say you go to an ice cream shop and you ask everyone there whether they like ice cream more or cake more.

Do you think the sample used (that is, people at an ice cream shop) is random?
It's not random at all. If they're at an ice cream shop, they probably like ice cream more.

And you find out that 80% of them like ice cream more. Is this result generalizable? (that is, can we say this applies to everyone?)
No! So it's not valid because the sample was not random.