Recent content by RichS

Great suggestions, thanks! I only know that the sample size is about 30% of the population, which is higher than I thought. I was surprised to learn that this large number of people actually have their Bluetooth device turned on. However, I don't know more details on how the averages are...

The system calculates the average time it takes vehicles to travel between two locations (Point A to Point B), and the calculation is done on 5-min intervals. I hope I explained better. :-)

In fact, I don't know for sure if these sub-samples are correlated or not. Let me explain a little more. Basically I'm looking at vehicle travel time between point A and B. These're collected by collecting MAC addresses from people's Bluetooth devices (mostly mobiles). Because of the...

Thanks very much He. You've been very helpful. I did some more testing on your formula. Sometimes it's good but sometimes it still produces a relatively large difference to the 'true' standard deviation. I think it's unavoidable as many other estimates. I also did some tests on using the...

Too bad, I thought I solved the problem but it's actually getting worse. In another little experiment I did, the estimated stdev is 300% of the 'true' stdev. :cry: I'm still struggling to understand this. Anyone has any ideas please?

I think I found the answer. It's not what I said. I'll post it after delivering my results which is due in a few hours time.

Thanks again He for your swift reply and your own testing. Does this provide any hint on where the problem is? https://www.physicsforums.com/threads/calculation-of-total-standard-deviation-over-samples-of-different-size.268377/ Below is a clip from this page. Maybe n1S12 is what's missing?

Hi mfb, I honestly thought your formula makes a lot of sense. However, when I just did some tests in spreadsheet it tends to underestimate standard deviation. Most of the time it's 60-80% of the "true" stdev but occasionally it's only 20%. I'm really puzzled by why this is. Could you help me...

Thank you mfb and mathman. mfb, I think your formula (sum sqrt(sample size)*abs(sample mean - total mean), divide by the square root of the sum of sample sizes) is close to what I've been looking for. I wanted to find a way to get the weighted average of Standard Error based on unequal sample...

Thank you both for your swift responses. Very much appreciated. I probably didn't explain clearly. Here's a made-up example of my data (sorry I don't have the real one with me right now): Sample Means (μ): 263, 343, 445, 655, 233, 324 Sample Size (n) : 34, 5, 76, 23, 43, 45 There's no other...

Gurus, can you help please? I've been given a set of samples, each has different sample size and mean (but not individual observations). I'm trying to figure out the population standard deviation so that I can estimate required sample size for certain confidence intervals. My question is how...