In Probability and Statistics, the standard deviation of a statistical population or data set is a measure of how much variation or dispersion exists from its average value. It is defined as the square root of the variance.
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Calculating $P(X-E(X)\geq \mathrm{SD})$ for a normal random variable $X$
If you'd have to calculate the following probability of an $X\sim \mathrm{Normal}$: $$P(X-E(X)\geq \mathrm{SD}),$$
how would you calculate it? And if the calculation will be for $4\cdot\mathrm{SD}$, ...
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984 views
What is difference between standard deviation and Z-score?
I have read several explanations of standard deviation and z-score, I know how to it calculate them but I am not sure what is differences betwen them.
Can someone explain it to me? When is suitable ...
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1answer
168 views
Percentile for a Given Z-Score
Disclaimer: Statistical math person I am not...
I'm a programmer that has been tasked with finding the percentile a child will fall in depending on their weight and age according to the CDC.
The ...
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2answers
262 views
Normal Distribution from Standard Deviation?
So I have a data set $(x_{1},y_{1}), (x_{2},y_{2}),\dots,(x_{n},y_{n})$ and from it I have the values of $\sum x$, $\sum x^{2}$, $\sum y$, $\sum y^{2}$, $\sum xy$.
My question is, how do I find a ...
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1answer
2k views
How to determine if Standard Deviation is high/low
I have derived the following response time data for a performance test I am running:
Min - 8sec
Max - 284sec
Average - 28sec
Standard Deviation - 27sec
What does the standard deviation say about the ...
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1answer
39 views
Probability of randomness from a given z-score
From a given z-score, what is the formula for calculating the probability of the deviation being due to chance?
e.g.
Given z = 1.2 standard deviations
The chance probability p = 0.11507
How did we ...
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1answer
72 views
Statistics, calculating mean and standard deviation
IF the mean of a set of scores is 80 and standard deviation is 10, explain the effect on mean and standard deviation in case following changes are made to each of the scores
1) adding 5
2) ...
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1answer
46 views
Bounding the standard deviation of a bounded random variable
Suppose $X$ is a random variable (discrete or continuous) whose values lie in the segment $[0,1]$. Is it safe to say that the standard deviation of $X$ is between $0$ and $\frac{1}{2}$?
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1answer
47 views
How to assure quality control to a 4% level of significance based on a 200 item sample with 24 defects?
For quality control purposes we have to accept or reject a large shipment of items. In order to do so we collect a sample of 200 items, 24 of which are defective. The manufacturer claims that at most ...
2
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1answer
89 views
Hypothesis testing for Variance - choosing a null hypothesis.
A gun-like apparatus has recently been designed to replace needles in administering vaccines. The apparatus can be set to inject different amounts of the serum, but
because of random ...
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2answers
156 views
Is it possible to calculate the mean and standard deviation from a median and quartiles?
any advice?, thanks!
I understand that the reporting of median and quartiles for small samples is an indication of skewed data. If such is correct, then trying to work out the mean and standard ...
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1answer
2k views
is it possible to calculate the standard deviation with a given mean and sample size?
I have been going in rounds with this problem... I may be thinking "complicated", any advice?
I have the mean and total sample size (=number of data points) and I need to know what is the standard ...
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2answers
84 views
Step by step correlation calculation
I must understand how I can calculate the correlation for the following probability variables.
...
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1answer
38 views
Population Standard Deviation
So we know that the population standard deviation $\sigma$ is just the square root of the variance $\sigma ^2$. But what are the uses of the standard deviation? At this juncture it just seems are ...
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1answer
168 views
Confidence Intervals: When to use what formula. I'm completely confused
I'm working on a few confidence interval questions, each with slight differences that make it very hard to determine what to use.
One question asks:
An electric scale gives a reading equal to ...
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1answer
359 views
Variance and Standard Deviation of multiple dice rolls
I'm trying to determine what the variance of rolling $5$ pairs of two dice are when the sums of all $5$ pairs are added up (i.e. ranging from $10$ to $60$).
My first question is, when I calculate the ...
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0answers
49 views
confidence interval
A telecommunication sales representative is responsible to sell various products such as wireless
equipment, satellite dishes and internet access. One particular sales representative working for
the ...
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1answer
39 views
Trouble with samples in a normal distribution
I'm okay with solving regular normal distribution questions (where X is a normal random variable with mean $\mu$ and standard deviation $\sigma$). However, we're currently dealing with samples within ...
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1answer
65 views
Finding probability 400 components last 5012 or less hours using standard deviation?
The lifetime of certain electronic components is a random variable with the expectation of 5000 hours and a standard deviation of 100 hours. What is the probability that the average lifetime of 400 ...
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1answer
288 views
Is standard deviation additive?
I am helping someone study for a statistics exam. I am quite good at most other math classes but it's been a while since I studied statistics. I am stuck on one of the exercise problems that we worked ...
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1answer
48 views
Standard error of a statistic
the standard error of a standard deviation is given as : s/n^(1/2). Would the standard error of kurtosis and skewness follow the same idea? For example, se of kurtosis = kurtosis/n^(1/2)?
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51 views
averaging STDs vs. calculating a single STD
I have a set of data points. I extracted the Standard Deviation for the whole set, and also extracted STDs for a partition of the points where each element of the partition has at least two points. I ...
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1answer
71 views
Relation of mean of standard deviations and standard deviation
Let $\{x_{i,j} : i=1..7,j=1,..n\}$ be a set of samples from $n$ weeks (where $i$ denotes the day of the week).
Is there any interesting information to be gleaned from the relationship (ratio, ...
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1answer
52 views
Moss scheduling Simulator and Standard Deviation in plain english?
source : http://www.ontko.com/moss/sched/user_guide.html
Configuration File Options standdev
n
*
The number of standard deviations from the average length of time a
process should execute ...
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0answers
86 views
rate of convergence of mean ,variance & skewness estimators
We are asked a question which of mean,variance or skewness converges faster. At first I thought it was straight forward answer: mean->variance->skewness. But I am not sure anymore because I read ...
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1answer
761 views
Average, standard deviation and min/max values
I'm analzying a computer science paper and just found in the experimental setup the following statement:
Average (standard deviation) of number of files per peer: 464 (554)
Min - max number of files ...
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2answers
129 views
flares probability
There are 33 no of flares distributed in 2 hemisphere north and south. the no of flares in north hemisphere is 14 and no of flares in southern hemisphere is 19. then how to find the probability of ...
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0answers
36 views
Standard Deviation of Infinite Size set
Given below are two sets of numbers. Both sets are infinite in size. Is the standard deviation of Set A greater than, equal to or less than the standard deviation of Set B ?
Set A is ...
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0answers
28 views
How to account for the difference in Sample standard deviation and Population standard deviation for a growth rate.
A question asked for the growth rate for a population of 500 bacteria growing for 2 hrs..In the equation $A(t) =$ $500\big(1 + (\frac{4t}{50+t^2})\big)$ where $t =$ time in hours and t is equal to ...
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2answers
1k views
Calculating missing data points from standard deviation and mean
I have to figure out what to missing data points are from a set of 10 scores. The mean of the 10 observed scores is 20.0 and the standard deviation is 6.0. The observed scores are listed below, with ...
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2answers
93 views
Mean and Standard Deviation
I've been struggling to solve this problem for quite some time. The solution states that the mean = 0,4% and SD = 2,5%, but I can't understand why.
"The daily return on a stock has a mean of 0,2% and ...
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2answers
2k views
How to interpret standard deviation of samples with different means?
Let be two samples collections A and B. Mean of the first is 5 and the second is 15. Standard deviation of the first is 2 and the second is 5.
Can we conclude something even though the two datasets ...
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2answers
358 views
Which is a better measure of uncertainty: standard deviation or resolution uncertainty?
Let's say I take a ton of measurements of the diameter of a marble with a 0.001" resolution micrometer, and I calculate the standard deviation of the sample set. Would the standard deviation be a ...
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1answer
128 views
Given a set of data (distance vs time) obtain a closer approximate for velocity
This is actually a physics experiment. I've collected a set of data
Distance and time it takes for a cart to move down an incline plane. From here, I've calculated the average time (...
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1answer
285 views
Standard Deviation vs Standard Error
I am confused about the difference between standard deviation and error. When do I use which? From my lecture:
Standard deviation quantifies spread of data about mean
Standard error measures spread ...
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1answer
69 views
Continuously sampled event: Estimating the value of a future data point, based on past measurements and their tendency
Problem
I'd appreciate some ideas on how to define a formula to estimate the value of a future data point for a continuously sampled event, based on past measurements and their tendency.
At any ...
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1answer
456 views
Meaning of Empirical formula when applied to some cases
When applying the Empirical formula on a data set, for example calculating 1, 2 or 3 standard deviations below and above the mean of a data set, for some cases, this might not make sense. For example, ...
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2answers
260 views
Adjusting mean and standard deviation
There's a set of 8 bags with the following weights in grams given:
1013, 997, 1013, 1013, 1004, 985, 991, 997
The mean is 1001.625, unbiased standard deviation is 10.86.
I have the following ...
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1answer
68 views
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1answer
31 views
extracting a function from a standard
I have a standard and I need create a function to resolve this, for example:
if my X < 21 my Y will be 24 else if my x < 28 my y will be 32 ...
how to a calculate a function for this?
thanks ...
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0answers
170 views
Aggregating standard deviation across many samples
I have data in the following form that correspond to data about sessions on a website: ID, # of actions (length), mean and standard deviation of time between actions:
...
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1answer
234 views
Statistics - Z-score confusion.
For z score, you are taking the sample value subtracting population mean and dividing it by std deviation. Is that correct so far?
Now, the "sample value" is defined by an equation. In my scenario, I ...
2
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2answers
179 views
Variance of a max function
Say $x_1$ and $x_2$ are normal random variables with known means and standard deviations and $C$ is a constant. If $y = \max(x_1,x_2,C)$, what is $\mathrm{Var}(y)$?
Well, I forgot to tell that $x_1$ ...
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0answers
61 views
Calculating the confidence interval
I've collected data such as mean $ $ and standard deviation $b$ values for several experimental studies, each of which was examined $n$ cases. I would like to report the results with the confidence ...
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1answer
664 views
normal distribution and standard deviation
1.in a normal distribution data the standard deviation is greater than the quartile deviation and the mean deviation ??
in a normal distribution 31% of the items are under 45 and 8% are over 64 find ...
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0answers
94 views
Local Standard Deviation characterization
Given the power spectrum of a correlated discrete random signal (e.g. non-white Gaussian noise), how is it possible to obtain an expression for its local standard deviation, i.e. the average standard ...
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0answers
760 views
Calculate standard deviation from ratio of two variables
Ok here is my problem. For my research I am investigating if we can use twitter to create sports data. One part of the problem is for example this:
We now for example that in the 6th minute of the ...
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1answer
474 views
Mean equal to standard deviation, with positive values
I read the question and the answers regarding the idea of the mean being equal to the standard deviation, in which the mean can always be adjusted to equal anything. But, if the data is restricted to ...
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1answer
2k views
When standard deviation is unknown?
I am reading a book about Statistics and I have encountered a text line:
...except in the case where standard deviation of the basic set is unknown...
I am not really sure what it means, could ...
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0answers
25 views
Policy/suggestion for determining when running standard deviation is becoming stable?
I have a program that simulates a computer system and keeping running average, variance, and standard deviation for bus traffic latency. I want to use the standard deviation to spot outliers, for ...
