Thread: t-test and p-value from processed data (mean, SD, sample number)

1. t-test and p-value from processed data (mean, SD, sample number)

Hi all,
I am new to this forum and would greatly appreciate your help. I know how to do a t-test for significance when I have simple sets of raw data. Now, however, I have to do it on already "processed" data, and in a more difficult way (at least for me).

What I have is the following: I have two sets of measurements of a parameter over time. For each time point, data from around 20 individual measurements were taken and processed to obtain the mean and the standard deviation; of course I also know the number of samples for each data point.

I would like to know how to perform a kind of t-test and how to calculate p-values to tell me whether the two sets of data ("curves" when plotted) are significantly different. In principle I think it should be possible to perform a t-test on the two data points (for which I have the mean, the SD and the sample size) for each time point.
But
(1) I don't know how to do a t-test on processed data, and
(2) I don't know how to get a (kind of) p-value for the two data sets over time, to show whether they are significantly different.

Could anybody help me with this problem?

2. You can "back into it" by squaring the SD to get the variance, then multiplying that by degrees of freedom (i.e., n1+n2-2) to get the sum of squares (SS), then using the SS in the t-test computation....

I have attached an Excel file that I've used to do this.

3. The Following User Says Thank You to JohnM For This Useful Post:

Squall (02-03-2017)

4. Originally Posted by JohnM
You can "back into it" by squaring the SD to get the variance, then multiplying that by degrees of freedom (i.e., n1+n2-2) to get the sum of squares (SS), then using the SS in the t-test computation....

I have attached an Excel file that I've used to do this.
thanks a lot, especially for the excel file. I have tested it on two data points from the sets, and it works great. I now have to go into the detail to understand what it does - your explanation is a bit too short for me but I think I can figure out the rest myself.
Now, is there any way to statistically evaluate whether the two "time curves" in total are significantly different? I could do the t-test for all individual time points (comparing two sets of data, "treated" versus "non-treated") - but then I end up with approximately 400 p-values (or "yes"/"no" values from your excel sheet). I am sure there is some great way to statistically handle these values, but I have no idea how...

5. hmmm, seems to be difficult. As it is not really a "homework"-related question (I just posted it here because I thought it is possibly too trivial for the more advanced parts of the forum), I consider to shift the second part of my question to the "statistical consulting" forum. Is that allowed?

6. Originally Posted by JohnM
You can "back into it" by squaring the SD to get the variance, then multiplying that by degrees of freedom (i.e., n1+n2-2) to get the sum of squares (SS), then using the SS in the t-test computation....

I have attached an Excel file that I've used to do this.
Thanks very much JohnM! That was very useful.

7. I am sorry to ask a silly question but how do you determine the SD if I only have the mean?

thanks

8. Re: t-test and p-value from processed data (mean, SD, sample number)

Hi all,

Very new to this forum and found the excel sheet posted by JohnM very useful for simplifying t-test calculations for my colleagues.

I was wondering if anyone could suggest to me how to adjust appropriate calculations within the file to allow an option to treat the data as paired or unpaired?

I am quite novice at stats (although I am very eager to learn), so I don't know if this is a ridicules query.

Many thanks to all.

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