Two Stats questions for Math nerds (std. deviation, mean, subsets)

AI Thread Summary
For the first question regarding a subset of 50 classes from a total of 3200, the expected average class size is approximately 64, with a standard deviation of 2. The second question involves estimating the average and standard deviation for samples of 40 and 160 classes, where the average remains at 48, while the standard deviations are approximately 1.9 and 0.95, respectively. The Central Limit Theorem is applied, indicating that larger samples will yield means that approximate the population mean, with decreasing standard deviations. Overall, the calculations confirm that both subsets maintain consistent averages while their standard deviations decrease with larger sample sizes.
ski
Messages
8
Reaction score
0
1. If a school offers 3200 separate courses and a survey of these courses determines that the class size is 50 with a standard deviation of 2, what would one expect for the average and standard deviation of a subset of 50 of these classes selected randomly?

2. In a survey to estimate the average size (number of students) of a class at a university, ten courses are picked at random and the class size of each is determined. The result is 48 students in a class with a standard deviation of 12 students. Assuming that the sample of 10 classes is representative subset of the whole and that class size is a Gaussian random variable, what would one expect the average and standard deviation to be for samples of 40 classes and 160 classes?

It's been so long since I've taken stats... REALLY long :confused: Any help on one or botha these would help me this week :smile:
 
Physics news on Phys.org
Here was what's suggested I do:

1.We'd expect an average of 50+- 1.98*2/sqrt(50)
The standard deviation by definition is 2/sqrt(50)

2. Not as sure about this one but following the logic of the last one..
sqrt(10)*12=s
and then stdv= s/sqrt(40)
and s/sqrt(160)
where the mean = STDV*2.021,STDV*1.97 respectively



Is the first solution really that easy?
 


1. For the first question, we can use the formula for standard deviation of a sample, which is s = √(∑(x - x̄)^2 / n-1), where s is the sample standard deviation, x is the individual class size, x̄ is the sample mean, and n is the sample size. In this case, n = 50 and we know the sample standard deviation is 2. We also know that the total number of courses is 3200, so the population mean would be 3200/50 = 64. Therefore, we can expect the average of the subset of 50 classes to be around 64 and the standard deviation to be around 2.

2. For the second question, we can use the same formula for standard deviation of a sample. In this case, n = 10, s = 12, and x̄ = 48. To estimate the average and standard deviation for samples of 40 and 160 classes, we need to use the Central Limit Theorem, which states that the sample means of large samples will follow a normal distribution regardless of the distribution of the population. This means that the average of the sample means will be the same as the population mean, and the standard deviation will be the population standard deviation divided by the square root of the sample size. Therefore, for a sample of 40 classes, we can expect the average to be around 48 (same as the population mean) and the standard deviation to be 12/√40 ≈ 1.9. Similarly, for a sample of 160 classes, we can expect the average to be around 48 and the standard deviation to be 12/√160 ≈ 0.95.
 
Thread 'Variable mass system : water sprayed into a moving container'
Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
Back
Top