Math Probability: Need to find the Variance

In summary, the average weight of plums in a large lot is 70 g with a standard deviation of 0.85 g. The variable X represents the average weight of plums in a pack of 14 plums. To find the variance of X, the formula Variance/n = 0.852/14 = 0.060208 is used, which gives the answer of 0.060208. It is important to note that the problem assumes that the weight of plums in the pack is independent, which may not always be the case in practice.
  • #1
CryptoMath
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Homework Statement


Weight of the plums of a large lot is averaging 70 g and SD 0.85 g.
X= the average weight of plums in a pack of 14 plums.

Find the variance of the variable X.

Homework Equations

The Attempt at a Solution


n=14
Average_Total= 70*14=980
SD2 = Variance_Total= 0.852 *12 = 10.115

Variance/n = 10.115/14 =0.7225

Does that make sense so far? I am pretty much stuck here.
 
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  • #2
You'll find it easier to understand if you write sentences around your formulas, explaining what they represent.
For instance, what is 'Average_Total' and why did you write
CryptoMath said:
Variance/n
I can easily guess the answers to these questions, but that's not the point. By writing it out, you clarify things in your own mind.

Writing explanatory sentences helps it fit together in a logical manner in your mind, rather than just being a jumbled bunch of formulas that you try to commit to memory, hoping to use the right one at the right time.
 
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  • #3
andrewkirk said:
You'll find it easier to understand if you write sentences around your formulas, explaining what they represent.
For instance, what is 'Average_Total' and why did you write

I can easily guess the answers to these questions, but that's not the point. By writing it out, you clarify things in your own mind.

Writing explanatory sentences helps it fit together in a logical manner in your mind, rather than just being a jumbled bunch of formulas that you try to commit to memory, hoping to use the right one at the right time.
Average_Total: as in the average of all the 14 plums.

Variance/n : To have the average variance I believe as this is what its asking in the problem
X= the average weight of plums in a pack of 14 plums.
and I need to find the variance.

But what distribution do I have to follow, is this binomial ?
 
  • #4
andrewkirk said:
You'll find it easier to understand if you write sentences around your formulas, explaining what they represent.
For instance, what is 'Average_Total' and why did you write

I can easily guess the answers to these questions, but that's not the point. By writing it out, you clarify things in your own mind.

Writing explanatory sentences helps it fit together in a logical manner in your mind, rather than just being a jumbled bunch of formulas that you try to commit to memory, hoping to use the right one at the right time.
I got this one resolved it is simply that:
Variance/n = 0.852/14 =0.060208, this is the answer.
Checked in my book to confirm :)
 
  • #5
That's right, the formula that the variance of a sum of independent random variables is the sum of the variances is true regardless of distribution.
BTW, the problem statement omitted to say that the weight of plums in the pack is independent, which is essential to obtaining the solution. In practice, that may not be the case, depending on how the sample of 14 was selected. For instance if there is a lot of jostling there will be more larger plums near the top, and smaller near the bottom. So 14 plums selected from near one another would be expected to have correlated sizes.
 
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  • #6
andrewkirk said:
That's right, the formula that the variance of a sum of independent random variables is the sum of the variances is true regardless of distribution.
BTW, the problem statement omitted to say that the weight of plums in the pack is independent, which is essential to obtaining the solution. In practice, that may not be the case, depending on how the sample of 14 was selected. For instance if there is a lot of jostling there will be more larger plums near the top, and smaller near the bottom. So 14 plums selected from near one another would be expected to have correlated sizes.
I have an other exercise, may you help me with that one please ?
 
  • #7
OK. Post it as a new thread though, unless it's very closely related, to avoid confusing readers..
 
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What is variance in math probability?

Variance is a measure of how spread out a set of data is. In math probability, it is used to determine the average distance of each data point from the mean, or average, of the data set.

How do I calculate the variance of a data set?

To calculate the variance of a data set, follow these steps:

  1. Find the mean of the data set.
  2. For each data point, subtract the mean from the data point.
  3. Square each of these differences.
  4. Find the average of these squared differences.
  5. This average is the variance of the data set.

Why is variance important in math probability?

Variance is important in math probability because it helps us understand the spread of data and how likely it is to deviate from the mean. It is also used to calculate other important measures, such as standard deviation and covariance.

What is the relationship between variance and standard deviation?

Variance and standard deviation are closely related. Standard deviation is simply the square root of variance. This means that they both measure the spread of data, but standard deviation gives us a more intuitive understanding of this spread as it is in the same units as the original data set.

How can I use variance in real-world situations?

Variance can be used in many real-world situations, such as finance, sports, and medicine. It can help us understand the risk and potential outcomes of investments, the performance of athletes, and the effectiveness of medical treatments. Basically, any situation where we want to analyze and make predictions based on data can benefit from the use of variance.

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