How to calculate p values for r values, in biserial correlation estimates from MPLUS

I have seen this issue twice in MPLUS forum. Once in 2003 for categorical outcomes and in some other time, related to LGC, the estimates of p values.

Linda Muthen explains r/SE is similar  to running a z test.

If you ask for TYPE=BASIC, you will get the correlations and also the standard deviations for each correlation. If you divide the correlation by its standard error, this is like a z-test.

This comment, can also be found in other references. the usual output of MPLUS follows the sequence:

                                  Two-Tailed
Estimatef S.E.g Est./S.E.h P-Valuei


And, according to Linda, Est./S.E. should be similar to Z. As I couldn’t find the formula to get the p value from a z score, I used this procedure: I got the same issue as Angela, and i resolve it by using excel.



taking this formula:



t = r * SQRT((n-2) / (1 - r*r))



source: http://publib.boulder.ibm.com/infocenter/db2luw/v8/index.jsp?topic=/com.ibm.db2.udb.doc/admin/c0006909.htm



source: http://faculty.vassar.edu/lowry/tabs.html#r



one can transform the r estimate into a t value. Then, to do test this excel formula is used:



p-value =TDIST(ABS(t),df,2) [where df are N-2, in a correlation]



To see if this was working, i compare the p value obtained by spss over two continuous variables vs, mplus estimates, and the results were fairly similar:



    r    N        t        P    
0,022    956      0,679    0,496    from MPLUS


0,022    956               0,491    from SPSS



Let me know if there is anything wrong or if you have any comments.