Extrapolating the meaning of a probability If I have 39% of students at a school that exhibit a specific, objective, measurable behavior, can I extrapolate this and say that any student at that school has a 39% chance of exhibiting that behavior?
 A: There is a 39% chance that a student selected at random from the entire population of students at that school exhibits the behavior.
Your wording/extrapolation is very slightly incorrect.
A: Let's say that the measurable behavior is gender. In other words, 39% of students at a school are female. If you picked out a student, let's call her Jill,  would you say Jill has a 39% of being female? No! She has a 100% probability of being female. However, if someone told you, "I'm going to pick a student out - what's the probability that they're female?" you ought to say 39%. 
Another example, if someone told you they were going to pick out a student who has shoulder length hair - what is the probability that they are female? Your answer should likely be higher than 39% (since a large proportion of those with shoulder length hair are female).
In other words, knowing absolutely nothing about the randomly selected student, they have a 39% of being female. But if you knew some other information, the probability changes. 
