Does Drain have a cap?

This is a continuation of a line of inquiry regarding the mechanics of Drain, specifically whether an "unconditional" maximum value of Drain exists (still thinking statistically...) and, if so, whether it can be achieved with a minimum level of dark magic skill such that additional skill does nothing to increase the unconditional maximum (holding all other relevant factors fixed, obviously).

To put it another way, does additional skill do nothing to increase Drain potency?

First, I supposed that a maximum Drain value could be achieved with a minimum dark magic skill of 300 (or somewhere around 300), and that 25 additional skill would therefore not change the average potency of Drain. Data were collected in the usual manner (taking care not to cast Drain with Dark weather in effect; see previous posts for more details on "experimental procedure"). All other factors affecting potency and or accuracy were held fixed for both samples. I obtained the following results:

The maximum Drain observed given 300 dark skill was 319 and the maximum observed given 325 dark skill was 318. Also note that there were more "obvious" resists under 300 skill than under 325.

Considering the data as a whole, does 25 additional dark magic skill really have no effect on Drain potency? Noting the lack of difference in maximum Drain values should be enough for most (and it is for me), but let us also consider a way to apply statistics.

First, we can treat this data as though we were doing equivalence testing, so we need to decide what average difference would be considered a sign that there is a difference in Drain potency.

Recall that last time, I observed that, given 269 dark magic skill, I observed a maximum Drain of 288. Here, I observed a maximum Drain of 319 given 300 dark magic skill. Does one point of additional skill really increase the maximum (and minimum) Drain by 1 HP? Who knows, but suppose it were the case. Then 25 additional skill would have increased the maximum (and average) Drain by 25 HP. Let us then consider any observed difference in averages between 0 and 25 to be the result of chance, provided that there really is no difference in potency. Of course, there are some observations that are obviously resists, so let us also use the crude cutoff that any values below 150 be excluded from the analysis.

By the logic of an equivalence test ("two one-sided tests"), a 90% confidence interval for the difference in average potency between 325 skill and 300 skill is (-7.003157, 19.665328). This confidence interval is completely contained in the interval [-25, 25], so from the standpoint of statistical significance one can say that 325 skill is equivalent to 300 skill as far as average potency is concerned.

Conclusions

There appears to be no difference in Drain potency between 300 dark magic skill and 325 dark magic skill, holding all other relevant factors fixed. Additional dark magic skill still appears to affect Drain accuracy beyond 300 dark magic skill, however. Drain potency could cap at 300 dark magic skill (holding other factors fixed).

Personally, these findings devalue somewhat my dark skill equipment that is devoted solely to Drain or Aspir, of which there are three pieces. Certain pieces such as Sorcerer's gloves and Wizard's tonban still provide large boosts to accuracy (at least it is expected).