New to Applied Statistics

balexmfl

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Jan 16, 2015
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1
I am taking Applied Stats in college. I haven't taken a math class for several years and was never any good with it to begin with. I am just not comprehending any of this. It is my first two weeks so we are working on:

Standard deviations
Variance of a random variable
Discrete probability for a random variable
Standard deviation of random variable

If someone can explain these things in the simplest of ways, if that's possible, I'd greatly appreciate it.
I will feel like I am starting to understand a formula and then my answers are wrong and I can't figure out what I have done. My professor is not very helpful, plus I am taking online classes so I am not really getting any hands-on assistance here…

Example problem that I am stuck on:

Construct a discrete random probability distribution for the random variable X

P(x,) = f,/N
N = (f,+f,…)

N = 4

X f
4 P(4) = 15/80 = 0.1875
5 P(5) = 11/80 = 0.1375
6 P(6) = 16/80 = 0.2
7 P(7) = 38/80 = 0.475

(The above answers were put in and came out being correct)

The second part...

Mean of the random variable X

Mean = Sum of [x * P(x)]

4(0.1875) + 5(0.1375) + 6(0.2) + 7(0.475)

= 0.75 + 0.6875 + 1.2 + 3.325

=5.9625

Now I am supposed to find the standard deviation of the variable x, which is apparently a different thing altogether and I am stuck. It gave me the formula:

Square Root Of {Standard Dev. of Variable X = Sum of [X2 * P(x)] - Mean of the random variable X (or the "sum of [x*P(x)] problem)}

When I tried this I got...

[42(0.1875) + 52(0.1375) + 62(0.2) + 72(0.475)] - 5.9625

= 3 + 3.4375 + 7.2 + 23.275

= 36.9125 - 5.9625

= 30.95

Square root: 5.5633


That was not right as the answer was supposed to be 1.2 (after rounding) and I just don't understand what I did wrong here….

Again, I am not really comprehending these as it seems the formulas change every time there is another question. I might just have trouble interpreting the question/answer but I am not sure.

--Bianca
 
I am taking Applied Stats in college. I haven't taken a math class for several years and was never any good with it to begin with. I am just not comprehending any of this. It is my first two weeks so we are working on:

Standard deviations
Variance of a random variable
Discrete probability for a random variable
Standard deviation of random variable

If someone can explain these things in the simplest of ways, if that's possible, I'd greatly appreciate it.
I will feel like I am starting to understand a formula and then my answers are wrong and I can't figure out what I have done. My professor is not very helpful, plus I am taking online classes so I am not really getting any hands-on assistance here…

Example problem that I am stuck on:

Construct a discrete random probability distribution for the random variable X

P(x,) = f,/N
N = (f,+f,…)

N = 4

X f
4 P(4) = 15/80 = 0.1875
5 P(5) = 11/80 = 0.1375
6 P(6) = 16/80 = 0.2
7 P(7) = 38/80 = 0.475

(The above answers were put in and came out being correct)

The second part...

Mean of the random variable X

Mean = Sum of [x * P(x)]

4(0.1875) + 5(0.1375) + 6(0.2) + 7(0.475)

= 0.75 + 0.6875 + 1.2 + 3.325

=5.9625

Now I am supposed to find the standard deviation of the variable x, which is apparently a different thing altogether and I am stuck. It gave me the formula:

Square Root Of {Standard Dev. of Variable X = Sum of [X2 * P(x)] - Mean of the random variable X (or the "sum of [x*P(x)] problem)}

When I tried this I got...

[42(0.1875) + 52(0.1375) + 62(0.2) + 72(0.475)] - 5.9625

= 3 + 3.4375 + 7.2 + 23.275

= 36.9125 - 5.9625

= 30.95

Square root: 5.5633


That was not right as the answer was supposed to be 1.2 (after rounding) and I just don't understand what I did wrong here….

Again, I am not really comprehending these as it seems the formulas change every time there is another question. I might just have trouble interpreting the question/answer but I am not sure.

--Bianca
You wrote above that to compute the Standard Deviation that you need X2 * P(x) and to compute the mean you need x*P(x). You had the x^2's and x's correct but your p(x)'s seem to be wrong.

You wrote
4 P(4) = 15/80 = 0.1875
5 P(5) = 11/80 = 0.1375
6 P(6) = 16/80 = 0.2
7 P(7) = 38/80 = 0.475

So why do you think that p(4)= 15/80, p(5)=11/80...?

What equals 15/80 (according to what you wrote) is 4*p(4), NOT p(4). To get p(x) from 4p(x) just divide my 4. So p(4)=15/320

Plug in the correct values of p(x) and you should be OK.
 
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