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The number of standard deviations 2014-06-15
From Dawn:
Suppose that, at a certain college, the average weight of all male students is 160 pounds with a standard deviation of 30 pounds. A certain male student weighs 145 pounds. Determine how many SD's his weight is above or below average.
Answered by Penny Nom.
Sample Standard deviation 2011-03-16
From Iris:
Help please: me on this type of math problem I can get through step one after that I get lost. Maybe you can explain it to me in a easier way to get the correct results. I have read the examples, but I am still lost cannot move on until the next practice until I get this one

Sample standard deviation
The following are distances (in miles) traveled to the workplace by employees of a certain computer company.

15,29,32 28 18 28
Find the standard deviation of this sample of distances. Round your answer to at least two decimal places.

Answered by Penny Nom.
Standard Deviation from a Proportion 2010-06-08
From silvestre:
A large company finds that approximately 17% of all purchases are returned for credit. If the company sells 100,000 different items this year, about how many items will be returned. Find the Standard Deviation.
Answered by Janice Cotcher.
Standard deviation 2009-10-27
From Reggie:
After you have computed the standard deviation of a set of 40 scores, you discover that the lowest score is in error and should be even lower. Will correcting this error affect the standard deviation?
Answered by Penny Nom.
Z-score 2009-03-25
From Barb:
I am having trouble finding information on a z score and the conversion to the number of standard deviations a z value can be away from the mean. What exactly does that mean and what am I looking for? Help Please.
Answered by Harley Weston.
Sigma in a normal distribution 2008-11-18
From Justin:
Suppose the random variable Y can be described by a normal curve with Mu=40. For what value of the standard deviation is P(20 less than or equal to Y less than or equal to 60) = 0.50


Answered by Harley Weston.
Finding the Raw Score for a Mean of 0 and Standard Deviation of 1 2008-10-02
From Anita:
A variable Y has a distribution of mean of Y =80 and standard deviation of Y=36. Find the values of A and B for transforming y*=A +BY so that mean of y* =0 and standard deviation of y*=1.
Answered by Janice Cotcher.
Expected value and standard deviation 2008-03-19
From Patrick:
Heres a question i cant figure out:
a small airline company has only three flights per day. The number of delayed flights per day is regarded as a random variable, and I'm supposed to calculate the expected value and standard deviation of the number of delays. the probability distribution looks like:
No. of Delays: 0 1 2 3
Prob. of delay: 05. 0.3 .1 .1

Answered by Harley Weston.
Mean and standard deviation 2007-12-10
From Jamie:
Let n be any odd whole number.
a) Which has the greater mean: the odd whole numbers from 1 to n, or the even whole numbers from 2 to n-1?
b) Which has the greater standard deviation: the odd whole numbers from 1 to n, or the even whole numbers from 2 to n-1?

Answered by Penny Nom.
A t-test 2007-11-20
From Rachel:
I am wanting to use the t-test to find if temperature over a period of 5 days was significantly different to the mean temperature for that time of year. So i want to compare expected temps to actual temps. The problem is that because the expected temperature is the same for the 5 days it gives me a standard deviation of zero. Does this mean the t-test cannot be used because the f-test can't be calculated. If so is there an alternative test?
Answered by Harley Weston.
Standard deviation 2007-11-20
From Shahzada:
when and why we use standard deviation in Data analysis?
Answered by Harley Weston.
Grouped data 2007-10-08
From Nimra:
The following sets of data represent the distributions of
house prices in the north-east and south-west of a country.
Random samples of 100 houses were selected in each region.
(a) Calculate the mean, median and standard deviation for

Answered by Harley Weston.
Standard deviation 2007-09-05
From Ethan:
Okay I have a question about standard deviation. Can you tell me a five number set of data that will have a standard deviation f zero and how you know that?
Answered by Stephen la Rocque.
Standard deviation 2007-09-01
From Pat:
I have looked over your math question and mines is not there. for a binomiial distribution with parmeter n and p, the mean is np and the standard deviation is np(1-p). I understand how to get the np but not understanding how to get the (1-p). For example np=(40)(0.48)(1-0.48)=3.160. How can I get the (1-0.48) answer please
Answered by Harley Weston.
A statistics example 2007-08-02
From Claudia:
A particular employee arrives to work some time between 8:00 am - 8:30 am. Based on past experience the Company has determined that the employee is equally likely to arrive at any time between 8:00 am - 8:30 am.

On average, what time does the employee arrive?

What is the standard deviation of the time at which the employee arrives?

Find the probability that the employee arrives exactly at 8:12 am?

Find the probability that the employee arrives between 8:20 am - 8:25 am?

Answered by Har.
Standard Deviation 2007-06-13
From Adrian:
If you are told that the mean salary of a certain group of workers is $30,000 with a standard deviation of $4000, what proportion of workers earn over $38,000? What proportion of workers earn less than $18,000? Assume the distribution of wages is normal.
Answered by Stephen La Rocque.
Normal distribution 2007-05-24
From Paula:
Consider a data set that is normally distributed. The mean of the data set is equal to 10,000.
a.) Suppose that, for this data set, 10,625 has a "z-value" = 2.5. Solve for the standard deviation of the data set.
b.) Solve for the "z-value" of 9,900.

Answered by Penny Nom.
Incremental standard deviation 2007-04-28
From Murtaza:
Hi In relation my question asked earlier, i would first like to thank you for the prompt reply. But it has not answered my query yet. Let me frame the problem again. I have a mean value for N data items. (note that i know N but i donot know the distinct N data item values) Now the (N+1)th data item comes in and i calculate the new mean. (incremental method for that is simple). How do i calculate the new standard deviation. The formula you gave me requires me to read the data items once and at the end of the pass i will have have the standard deviation. But i need the STDDEV values at intermediate stages as well. Is it possible ? Murtaza
Answered by Penny Nom.
A one pass calculation of standard deviation 2007-03-19
From Murtaza:
Hi I searched the listings but couldn't find what i am looking for. I have a dataset and i am allowed only 1 pass through it. At the end of this pass i must have the mean and standard deviation. Calculating the mean is easy as also demonstrated in your listings. I think the same can be done for standard deviation as well. But the doubt that i have is this :- Will the calculated standard deviation be sequence dependent ? i.e. if i change the order in which the data items enter the system, will the std deviation also change ?? Thanks in advance Murtaza
Answered by Penny Nom.
Standard deviation 2007-01-15
From Kayleigh:
After i have added up all the squared deviations what do i do???, what is meant by "divide by one less than the sample size"
Answered by Stephen La Rocque.
Standard deviation 2006-10-04
From Jennifer:
I was wondering how to figure out a standard deviation if you only have the mean of the data and no other information?
Answered by Stephen La Rocque.
Absolute Value vs. Standard Deviation ? 2006-04-24
From Sonia:
Why we don't just take the average of the absolute value of difference scores (use the mean deviation) to describe variability instead of calculating the standard deviation?
Answered by Claude Tardif.
Means of sample means 2005-10-12
From Fareeha:
A population consists of six numbers 1,2,3,4,5,6
a) How many different samples of two numbers can be selected with replacement? and list them.
b) Calculate the mean and standard deviation of the population
b2) Calculate the mean and standard deviation of the mean of the samples.
c) Find the standard error of your estimated mean in question b2 and compare with the estimated standard deviation in b2.

Answered by Penny Nom.
A normal distribution problem 2005-08-08
From Brad:
The life of a toy is normally distributed. Suppose 92.51% of the items lives exceeding 2,160 hours and 3.92% have lives exceeding 17,040 hours. Find the mean and the standard deviation.
Answered by Penny Nom.
Standard deviation 2005-01-02
From Urial:
Find the standard deviation of the given data set
To get the best deal for CD dealer, Jessica called eight appliance stores and asked the cast of a specific model. The prices she quoted are listed below.

$300, $203, $272, $332, $440, $119, $129,$254

Answered by Penny Nom.
A standard deviation test 2004-11-09
From Karen:
How do you compare to see if a sample standard deviation is different than the population standard deviation? I know how to compare means, but not standard deviations.
Answered by Penny Nom.
Incremental variance 2004-09-23
From Carlos:
I must keep statistical data (mean and variance) in 3 granularity levels depending on the age of the data (daily for older than 1 year, hourly for older than 1 month and quarter-hour for older than 1 day). How can I calculate the resulting variance from a set of variances previously calculated supposing I have the count and mean for each member of the set?
Answered by Andrei Volodin and Penny Nom.
Statistics from grouped data 2003-10-09
From Rebecca:

I have to use a suitable computer package (ie. Excel) to work out the mean and standard deviation of the following:

 Price Range            No of Houses        No of Houses 
`000                  Area 1              Area 2   

45 and under 50         4                   5 
50 and under 55        7                   12 
55 and under 60        14                  16 
60 and under 70        19                  18 
70 and under 80        8                   3 
80 and under 100       5                   2 

I have used excel to work out mean and standard deviation before, but not in a more complicated example like this.

Any advice on where to start would be greatly appreciated.

Answered by Penny Nom.
Standard Deviation 2003-10-07
From Rebecca:

I have a task to complete, which is to calculate the mean and standard deviation of something. I have done this but am then asked to write a short explanation of my findings.

I know what the mean is about, and I thought I knew what the standard deviation meant too - shows the variation from the mean. However, on a task I completed earlier the feedback I got said 'you need to tell us that it is talking about the middle 66% of the data' - that has thrown me, I don't understand that. Can anyone help me get my head round this???

Answered by Penny Nom.
Incremental standard deviation 2003-04-12
From Carlos:
I need to calculate the standard deviation for a group of data, but I don't know in advance what is the mean. Is there a way to adjust the STDV for each datum without keeping all of the previous values? This is needed basicaly for performance, so I won't need to read twice the same data (spend processing time) nor save the previous values (spend memory).
Answered by Andrei Volodin and Penny Nom.
Variance 2002-10-29
From Ade:
I am trying to solve a standard deviation question, I have 30 data points to use, should I group the data before solving for variance or should I just use the raw data.
Answered by Andrei Volodin.
Range of values for standard deviation 2002-03-22
From Susan:
What is the range of values for standard deviation? Is it between 0 and +3? Is it between +1 and +3?
Answered by Andrei Volodin.
Can the standard deviation ever = 0? 2002-02-16
From Karen:
If given a class of 30 people who take a test with a mean of 80. Can the standard deviation ever = 0? If so, why?
Answered by Penny Nom.
Ciefficient of variation 2001-09-23
From Carmen:
I have a question from my OAC finite class. I've come across a problem with the coefficient of variation. I have taught my students that there are no units for coefficient of variation and it can be expressed as a percent. So, for example, a set of data with mean of 5 and standard deviation of 100 would have a CV of 5%. But what happens in this situation: the mean is 4meters and the standard deviation is 0.7mm. Is the CV 1.75% or 0.00175% or 0.0175%? I've had some students change leave the units as is, change them both to mm or change them both to meters...so which is it and why?
Answered by Penny Nom.
Standard deviation 2001-09-22
From Doug:
I was wondering if anybody knows who the founder of standard deviation was?
Answered by Penny Nom.
Comparing means and standard deviations 2001-07-11
From Shelly:
Is there a way to compare means and standard deviations that come from different size samples? Is there an equation that can normalize sample size?
Answered by Andrei Volodin.
Sigma 2001-03-31
From A student:
What does the Sigma symbol mean in mathematics?
Answered by Penny Nom.
Sample variance 2000-04-16
From Jonathan Freeman:
I was just reading your article entitled "A Note on Standard Deviation" I'm now teaching a unit on s.d. and my students were wondering why one uses a denominator of n for a population and n-1 for a sample. I saw in your article that this is because "[the quantity] tends to underestimate sigma... and other technical reasons." To which my students again asked... "Why?" Could you please elaborate a bit on the "other technical reasons" perhaps in terms a high school senior (or their teacher...) could understand?
Answered by Harley Weston.
Absolute value 1999-12-29
From Alex Waller:
What are some practical applications for absolute value?
Answered by Harley Weston.
Estimating the population mean 1999-11-13
From John Barekman:
Statitistics: Estimating the population mean when the standard deviation is known:

I am not sure which n to use in the formula for the confidence interval equation:

x +/- z*(standard deviation/sqrt(n))

If we have data of ten people, and if we have the data of ten sets of ten people each, what is the difference in the n that we use? What is the difference between the standard deviation and the standard error? Are we using the number of sampling means or just the number of samples?
Answered by Harley Weston.

Standard Deviation 1999-07-11
From Anthony Fama:
I have seen several answers to this question: If one standard deviation represents 68% of the population, what does two, three, four and five sigma [std deviation] represent? As stated, I have seen several different answers and thus, the impetus for my question.
Answered by Harley Weston.
Mean and Standard Deviation 1998-04-22
From Kit:
i have a problem of maths.
i think the level of the question is secondary. question: Find the mean and the standard deviation of the numbers:1, 3, 4, 5, 7. i hope u solve this question.
thank you so much!

Answered by Harley Weston.



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