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 Oct 11 accepted By convention $P[X=x] = 0$ for all x. How would you explain pdf $f(x) =3x^2$ (where x is between 0 and 1) when x =0.9 Oct 11 comment By convention $P[X=x] = 0$ for all x. How would you explain pdf $f(x) =3x^2$ (where x is between 0 and 1) when x =0.9 @AndréNicolas i understand that part. but do you know what does $f(0.9)$ yield? Oct 11 comment By convention $P[X=x] = 0$ for all x. How would you explain pdf $f(x) =3x^2$ (where x is between 0 and 1) when x =0.9 @chris ok, i see. then what do i get when I plug in 0.9 for x? Oct 11 revised By convention $P[X=x] = 0$ for all x. How would you explain pdf $f(x) =3x^2$ (where x is between 0 and 1) when x =0.9 added 3 characters in body Oct 11 comment By convention $P[X=x] = 0$ for all x. How would you explain pdf $f(x) =3x^2$ (where x is between 0 and 1) when x =0.9 ok, when $x =0.9$ then $f(0.9)= 3(0.9)^2 which \neq 0$. That's what I'm trying to ask Oct 11 asked By convention $P[X=x] = 0$ for all x. How would you explain pdf $f(x) =3x^2$ (where x is between 0 and 1) when x =0.9 Oct 10 revised How can gender and class classification be dependent? edited title Oct 10 comment How can gender and class classification be dependent? Ross, thank you for your answer! I upvoted everyone who contributed, honestly I still don't quite get it. but it's pretty late tonight. I'll think more on each answer when i wake up tomorrow morning. Oct 10 comment How can gender and class classification be dependent? oh no =( this is so convoluted. I started a post before on how I can identify a probability problem as a conditional probability problem(math.stackexchange.com/questions/205103/…). it seems like you're suggesting they're the same. this is still confusing =( Oct 10 comment How can gender and class classification be dependent? why would you think of this in terms of conditional probability? I'm thinking it's simple union and interects Oct 9 revised How can gender and class classification be dependent? deleted 7 characters in body Oct 9 comment How can gender and class classification be dependent? If, however, the first die landed on 6, we would be unhappy because we would no longer have a chance of getting a total of 6. In other words, our chance of getting a total of 6 depends on the outcome of the first die; thus, E1 and F cannot be independent. So is there an explanation like this for class and gender? Oct 9 revised How can gender and class classification be dependent? deleted 46 characters in body Oct 9 revised How can gender and class classification be dependent? deleted 13 characters in body Oct 9 comment How can gender and class classification be dependent? @QiaochuYuan Suppose that we toss 2 fair dice. Let E1 denote the event that the sum of the dice is 6 and F denote the event that the first die equals 4. Hence, E1 and F are not independent. Realistically, the reason for this is clear because if we are interested in the possibility of throwing a 6 (with 2 dice), we shall be quite happy if the first die lands on 4 (or, indeed, on any of the numbers 1, 2, 3, 4, and 5), for then we shall still have a possibility of getting a total of 6. Oct 9 revised How can gender and class classification be dependent? deleted 4 characters in body Oct 9 asked How can gender and class classification be dependent? Oct 9 accepted Why does negative binomial random variable uses ${n-1 \choose r-1}$ instead of ${n \choose r}$ as coefficient? Oct 7 accepted Why can we use poisson distribution in this case? the n is very small!! Oct 7 comment Probability given a mean and standard deviation of a random variable @DilipSarwate had the distribution been normal, i'd have been right?