Need help with statistics

Factual Questions
1

…specifically the t-test. My dissertation project has been to expose cancer cells to a range of concentrations of a compound, generating a dose-response curve. I can calculate mean and sd of the replicates till the cows come home, but now I need to show a statistical difference between two concentrations on the curve. Statistics class was decades ago, and my book assumes I’ve read it from page 1 when I get to the t-test part (p. 315). I’m using Excel to calculate the t-test, and my questions are:

  1. should I use a one- ot two-tailed test? I was thinking of a two-tailed test for greater stringency.

  2. I assume I should do a paired test, as opposed to a homo- or heteroscedastic test, right? I am comparing two sets of the same number of data points from the three experiments done in triplicate (so N = 9).

  3. how do I determine what p value to indicate? p<0.01, 0.05, 0.001? I was never clear as to what degree of separation was indicated by the notation.

Thanks in advance.

Vlad +/- Igor, p<certain.

2

Noodge. 57 views and no one can help me? There’s gotta be someone out there who knows how to calculate p values.

3

Um, I’m not exactly a statistics expert, but my gut responses to your questions are:

  1. Yes, two-tailed. (In the social sciences that’s the default unless you have some compelling reason for using a one-tailed test.)

  2. From your description of the project, I don’t think this sounds like a paired sample. You need to have pairs of individual data points, not just two groups of data.

  3. Can’t you use a statistical program that will give you p-values? Stata or SPSS or something? I don’t have much SPSS experience, but I know you can just import a csv file and it’s not difficult to run t-tests.