
1. Re: TTest calculation
Joe Mako May 20, 2011 11:19 PM (in response to Steve Wexler)best to provide details on the calculation you want to perform.
From looking at http://www.socialresearchmethods.net/kb/stat_t.php I am not sure how the ttest concept described there fits with your description.
If you can provide your logic, and sample data, a calculated field can be provided.

2. Re: TTest calculation
Joe Mako May 20, 2011 11:56 PM (in response to Steve Wexler)I also found http://www.jerrydallal.com/LHSP/psd.htm and with both those references, I was able to make the attached.
Is this what you are looking for?

proportion_tvalue.twbx 32.7 KB


3. Re: TTest calculation
Steve Wexler May 22, 2011 5:05 AM (in response to Steve Wexler)Joe,
I had a feeling either you or Andy would pipe in on this quickly.
A colleague gave me a simple spreadsheet that shows how in Excel to compute the TTest. To be honest, I'm not sure if it is correct but if it is then your approach needs a couple of modifications.
Have a look at the attachment (cells A1 and B1 contain the response counts).
Thanks.
Steve

simplified_ttest.xlsx 9.5 KB


4. Re: TTest calculation
Joe Mako May 22, 2011 7:34 AM (in response to Steve Wexler)after a little bit of searching, I think what you are looking for is called the twoproportion ztest (at least the Excel formula used matches the results).
I did the calculation two ways in the workbook, one calc field for each concept, and another as a single calc.

zscore.twbx 38.7 KB


5. Re: TTest calculation
Steve Wexler May 22, 2011 8:35 AM (in response to Steve Wexler)Joe,
I forgot to mention how much I like the sum(1) trick. I had been using countd(ID), but really like the simplicity of sum(1).
This looks right to me, but I need to find out if this is what is wanted or the ttest (this is not my area of expertise, but a great opportunity to learn new stuff.)
Also, you are correct that the values should be negative.
Very cool (and thanks for doing both the single and dual calc versions).
Steve

6. Re: TTest calculation
Joe Mako May 22, 2011 8:45 AM (in response to Steve Wexler)SUM(1) is the same thing as SUM([Number of Records]), it feels like less typing/clicking for me, just a personal preference.
Whereas COUNTD([ID]) is not always the same as SUM(1), but I can imagine situations where they would be the same, it depends on your data and situation.
Here is the reference that I used, they provide some sample situations/questions as well:
http://stattrek.com/APStatistics4/TestDifferenceProportion.aspx?Tutorial=AP

7. Re: TTest calculation
Steve Wexler May 22, 2011 9:14 AM (in response to Steve Wexler)Joe,
Got you on the SUM(1) thing.
Thanks for the link and your generosity.
Steve

8. Re: TTest calculation
rochelle.sun Jul 13, 2012 3:05 PM (in response to Joe Mako)Hello Joe.
I have a similiar question about Ttest for "mean", rather than proportional.
Do you know how to create same calculation in Tableau for the excel function T.Dist.2T(% lift from mean of treatment group vs. mean of control group, degree of freedom)?
Thanks.
Rochelle

Ttest_significance.xlsx 11.1 KB


9. Re: TTest calculation
Joe Mako Jun 17, 2012 9:20 AM (in response to rochelle.sun)Rochelle,
Can you please provide a sample Excel file that performs the calculation you are looking for?
Also I recommend starting a new thread

10. Re: TTest calculation
rochelle.sun Jul 13, 2012 3:05 PM (in response to Steve Wexler)Hi Joe.
I attached an excel file with sample calculation in my post above. Basically I need a similiar calculation in Tableau to achieve the T.DIST() function in excel to get pvalue of a Ttest.
I will start a new thread too.
Please let me know if you have any insights. Thanks!
Rochelle