Linked Questions

69
votes
4answers
140k views

Does the variance of a sum equal the sum of the variances?

Is it (always) true that $$\mathrm{Var}\left(\sum\limits_{i=1}^m{X_i}\right) = \sum\limits_{i=1}^m{\mathrm{Var}(X_i)} \>?$$
8
votes
1answer
28k views

Standard Deviation After Subtracting One Mean From Another

I've got two sets of data from some fluorescent cells. The first set is when the cells don't have their fluorescence switched on, but they are still faintly glowing. The second set is when they do ...
10
votes
1answer
5k views

“Averaging” variances

I need to obtain some sort of "average" among a list of variances, but have trouble coming up with a reasonable solution. There is an interesting discussion about the differences among the three ...
1
vote
1answer
599 views

Calculating standard deviations for aggregated means

Example: Say I have a normal distribution that contains responses from both male and female participants with an average score of 50 ± 2. Lets then say I actually receive this information in the ...
0
votes
1answer
340 views

“Average” standard errors of multiple samples

let's assume that I have 3 subjects with each of them producing 5 values. Now, I can calculate the mean, variance, standard deviation, and standard error for each subject. Each subject has its own ...
1
vote
0answers
406 views

Sample standard deviation or population standard deviation

I'm running an A/B testing in a website and I'm capturing the number of clicks in a given area of a page. I am calculating the average of clicks per user's session and its standard deviation. The ...
0
votes
0answers
398 views

Calculate AttrakDiff confidence rectangle

I evaluated an app on attrakdiff.de where I got this chart. It shows PQ and HQ. It ranges in both directions from -3 to +3. In the chart is a point (PQ, HQ) which is surrounded by a confidence ...
0
votes
1answer
202 views

Average of values, their standard deviations and ranges

After reading for hours, I am still swimming. I have a set of plasma concentration results from different labs: ...
0
votes
0answers
197 views

Sum of Variance or Standard Error?

My end goal is to present the average travel time for a corridor and the standard deviation (or standard error) of that travel time. I'm not sure what to present to be statistically correct, here is ...
0
votes
0answers
68 views

Is it valid to average IQR values?

I am interested in quantifying the dispersion of pixel values in one image. More specifically, I want to get an average value of dispersion of the image column-wise (i.e., compute the dispersion of ...
0
votes
1answer
43 views

Estimating the number of people in an known area by sampling a smaller area

here's the thing I've trying to wrap my head around. Please point out any error in reasoning. Imagine you want to estimate the number of people in a field of known total area ($A_{T}$). People are ...