st: How to generate an index for the calculation of geometric mean

[Yuval Arbel <yuval.arbel@gmail.com>]

Dear Statalisters,

I would like to generate an index variable for the calculation of the
geometrical mean of an unbalanced panel (namely, the exponent, which
makes the product a
geometrical mean).

Here is an example of one panel:

. list reduct_per y3 if appt==2862

        +---------------------+
        | reduct~r         y3 |
        |---------------------|
    30. |        1          1 |
    31. |        1          1 |
    32. |        1          1 |
    33. |        1          1 |
    34. |        1          1 |
        |---------------------|
    35. |        1          1 |
    36. |        1          1 |
    37. |        1          1 |
    38. |        1          1 |
    39. |        1          1 |
        |---------------------|
    40. |        1          1 |
    41. |        1          1 |
    42. |        1          1 |
    43. |        1          1 |
    44. |        1          1 |
        |---------------------|
    45. |        1          1 |
    46. |        1          1 |
    47. |        1          1 |
    48. |        1          1 |
    49. |        1          1 |
        |---------------------|
    50. |        1          1 |
    51. |        1          1 |
    52. |        1          1 |
    53. |        1          1 |
    54. |        1          1 |
        |---------------------|
    55. |       25         25 |
    56. |       25        625 |
    57. |       25      15625 |
    58. |       25     390625 |
    59. |       25    9765625 |
        |---------------------|
    60. |       25   2.44e+08 |
    61. |       25   6.10e+09 |
    62. |       25   1.53e+11 |
    63. |       25   3.81e+12 |
    64. |       25   9.54e+13 |
        |---------------------|
    65. |       25   2.38e+15 |
    66. |       25   5.96e+16 |
        +---------------------+


 The problem is that for reduct_per==1 I would like
the index to become a constant and remain the same until the next
reduct_per, which is different from zero, comes.

Consider the example above: during the first 26 periods the index
should be one and and starting from the 26th period it should change
to 2,3 etc.

If, on the other hand, we would have opposite descending trail (namely
25 percent reduction rate from 1-25 and zero starting from the 26th
period), the index should be 1, 2,..,25 and then becomes a constant 25 from
the 26th period.

What is the simplest way to generate such an index? Can it be done
without a loop?

-- 
Dr. Yuval Arbel
School of Business
Carmel Academic Center
4 Shaar Palmer Street,
Haifa 33031, Israel
e-mail1: yuval.arbel@carmel.ac.il
e-mail2: yuval.arbel@gmail.com
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