HSEM 3010 notes 003    January 30, 2007

When to Use Which Statistical Test?
-----------------------------------

  A number of statistical tests are used to evaluate the results of
clinical trials.  How do you tell which one you should use?

  It depends primarily on the design and the endpoint (also known
as, the criterion outcome).

  1.  If the outcome is dichotomous (e.g., yes or no, died or stayed alive)
      and if there are two randomization groups, the analysis is that which
      is appropriate for a 2 x 2 table: either the Yates-corrected chi-square
      test or the Fisher Exact Text.  Of these two, the Fisher Exact Test
      (two-sided) is preferred, though as a rule they give very similar results.
      See notes 001.  You can use the website that is referenced in notes 001
      to carry out the computations.  The correct degrees of freedom for a
      chi-square test for a 2 x 2 table is: DF = 1.

  2.  If the outcome has 3 or more categories (e.g., got better, stayed the same,
      got worse) you can use a chi-square test for an R x C table.  Here R
      denotes the number of rows in a table (see example below, where R = 3),
      and C denotes the number of columns.  The degrees of freedome for this
      test is DF = (R - 1)*)(C - 1).  For the table below this is 2 * 2 = 4.


                    Treatment A     Treatment B     Treatment C
                  -----------------------------------------------
                  |               |              |              |
      Got Better  |       22      |      35      |      28      |
                  |               |              |              |
                  -----------------------------------------------
                  |               |              |              |
      Stayed Same |       30      |      44      |      15      |
                  |               |              |              |
                  -----------------------------------------------
                  |               |              |              |
      Got worse   |       34      |      41      |      12      |
                  |               |              |              |
                  -----------------------------------------------