Probability of 0-6 Students in Class of 20 - Help!

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Discussion Overview

The discussion revolves around calculating the probability of having 0 to 6 students in a class of 20 who fall into a specific category, given that 8% of students are in that category. The scope includes probability theory, specifically the binomial distribution, and touches on related concepts such as expected value and graphical representations of probability distributions.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant states the need to calculate the probability of having 0 to 6 students in a specific category using the binomial distribution formula.
  • Another participant explains the binomial distribution and provides the formula for calculating the probability of exactly i successes in n trials, noting the probabilities of success and failure.
  • A participant expresses confusion about certain notation in the equations, specifically mentioning "frac" and "tex".
  • Another participant suggests that if there were 100 students, the chance of exactly 8 students being in the category would be 100%, but the chances of having 7 or 9 would be less than 100%, proposing a graphical representation of probabilities.
  • One participant corrects the misunderstanding about the probability of having 8 students in a larger class, emphasizing that there is always some probability that none or all students are in the category, regardless of class size.
  • There is a mention of the expected value for the binomial distribution and a clarification that the shape of the probability distribution is bell-shaped, not parabolic.
  • Participants discuss issues with LaTeX formatting in their equations and share experiences of making similar mistakes.

Areas of Agreement / Disagreement

There is no clear consensus on the graphical representation of probabilities or the implications of class size on the probability of students being in the category. Some participants agree on the use of the binomial distribution, while others express differing views on the interpretation of probabilities in larger classes.

Contextual Notes

Some participants express uncertainty about the notation and mathematical steps involved in the binomial distribution calculations. There are also unresolved questions regarding the graphical representation of the probabilities and the nature of the distribution curve.

dranger35
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I haven't done a probability problem in a long time. Thank you.

Assume that 8 % of the students fall in some particular
category. We have 20 students in our class. What is the
probability that we have 0, 1, 2, 3, 4, 5, or 6 of those students in
our class?
 
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Binomial distribution. If the probability of a "success" in one "trial" is p, then the probability of a "failure" is 1-p. The probability of exactly i "successes" in n "trials" (and so n-i "failures") is [tex]_nC_i p^i q^{n-i}[/tex] where [tex]_nC_i[/tex] is the "binomial coefficient" [tex]\frac{n!}{i!(n-i)!}[/tex]).

The probability that anyone student is in that category is 0.08 so the probability a student is NOT in that category is 0.92. The probability exactly i students out of 20 are in that category is [tex]\frac{20!}{i!(20-i)!}i^{0.08}(20-i)^{0.92}[/tex].

Calculate that for i= 0, 1, 2, 3, 4, 5, 6 and add.

(Sorry, I left out the "[ tex ]" originally)
 
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Ok

Thanks a lot I needed it
 
Wait

What did you mean by frac in the beginning of the equation and tex in tha last part.?
 
dranger35 said:
What did you mean by frac in the beginning of the equation and tex in tha last part.?
I think it's just a typo. The correction would be [tex]\frac{20!}{i!(20-i)!}i^0.08(20-i)^0.92[/tex]
 
I don't know anything about probability but I would like to and I just had a thought.

If there were 100 students then the chance of 8 students being in that category would be 100% no? However, the chances of 7 students or 9 would being in that category would be lass than 100%. So their is a maximum and and it seems you could graph the probabilities verses the number of students and it would be parabola?
 
I think that's the idea of the bell curve.
 
dranger35 said:
What did you mean by frac in the beginning of the equation and tex in tha last part.?

I accidently left out the beginning "tex" tag. I edited to fix that.

honestrosewater: It's good to know I'm not the only one who messes up latex! (Or did you do that intentionally to make me feel better?)

You need { } in "i^{0.08}"

slug:"If there were 100 students then the chance of 8 students being in that category would be 100% no?"

No, if the probability of a student being in a certain category is any number less than 1.00, no matter how many students you have in a class, there is always some probability that NONE of the students are in that category and some probability that ALL are. You are, however, correct that the probability is highest at the "expected value" which, for a binomial distribution is np. In the orginal problem that is (20)(0.08)= 1.6 (round to 2) and if n= 100, 8.
But it's not a parabola (for one thing probability is never negative!)- it is, as philosophking said, a bell-shaped curve: basically given by [tex]e^{-x^2}[/tex].
 
HallsofIvy said:
honestrosewater: It's good to know I'm not the only one who messes up latex! (Or did you do that intentionally to make me feel better?)

You need { } in "i^{0.08}"
Actually, I just copy-pasted what you had written and added the beginning [ tex ]. Oddly enough I did wonder if that was correct- but you had just written [tex]_nC_i p^i q^{n-i}[/tex] correctly so figured you knew to include the braces. Funny. I do mess up though- that's why I preview before posting. :biggrin:
 

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