Power Repeated - Error

[Martyn Sherriff]

I am using power repeated for a sample size calculation. It is successful in calculating the 'between effect' and 'between–within effect' sample size, but fails with the 'within effect'

. matrix M = (0.415,0.496,0.549,0.588,0.887,0.881\        ///
> 0.91,0.983,1.011,1.645,2.421,2.917\                     ///
> 0.938,0.814,0.735,0.641,0.599,0.71\                     ///
> 0.754,0.848,0.842,1.112,1.796,2.635)

. matrix C = (0.513,0.308,0.308,0.308,0.308,0.308\                ///
> 0.308,0.513,0.308,0.308,0.308,0.308\            ///
> 0.308,0.308,0.513,0.308,0.308,0.308\            ///
> 0.308,0.308,0.308,0.513,0.308,0.308\            ///
> 0.308,0.308,0.308,0.308,0.513,0.308\            ///
> 0.308,0.308,0.308,0.308,0.308,0.513)


. power repeated M, covmatrix(C)

Performing iteration ...

Estimated sample size for repeated-measures ANOVA
F test for between subjects
Ho: delta = 0  versus  Ha: delta != 0

Study parameters:

        alpha =    0.0500
        power =    0.8000
        delta =    0.7143
          N_g =         4
        N_rep =         6
        means =   <matrix>
        Var_b =    0.1746
       Var_be =    0.3422
          Cov =   <matrix>

Estimated sample sizes:

            N =        28
  N per group =         7


. power repeated M, covmatrix(C) factor(bwithin)

Performing iteration ...

Estimated sample size for repeated-measures ANOVA
F test for between-within subjects with Greenhouse-Geisser correction
Ho: delta = 0  versus  Ha: delta != 0

Study parameters:

        alpha =    0.0500
        power =    0.8000
        delta =    1.9218
          N_g =         4
        N_rep =         6
        means =   <matrix>
       Var_bw =    0.1262
      Var_bwe =    0.0342
          Cov =   <matrix>
    spherical =      true

Estimated sample sizes:

            N =        12
  N per group =         3


. power repeated M, covmatrix(C) factor(within)

Performing iteration ...
failure to compute sample size;
    The computed initial value for the search algorithm failed. It is likely that the estimate is not achievable given the input parameters and the domain limits of the power function.
r(498);

I would be grateful for any suggestions on how to resolve this problem.

Thank you,
Martyn

Stata 15.1 (IC) Current update level:27 Jun 2018