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I've just discovered "calculated questions" in Moodle and I'm trying to create a simple one where I would be asking the student to find the probability that an observation from a standard normal population be greater than variable $\left\{z\right\}$.

I then define the range of $\left\{z\right\}$ as $[-3,3]$ with $2$ decimal places.

Now since Moodle doesn't have erf implemented I have been trying to emulate it using Jack D'Aurizio's answer here, and otherwise Abramowitz and Stegun's first approximation given here, which results in $$P(z_i> z)\approx \frac{1-\sqrt{1-e^{-0.619495805z^2}}}2$$ for the former, for instance.

I am facing the problem that Moodle refuses to calculate this when $z$ is less than $2$ (I get the output NaN) and as for the values where it will output something, the outputs don't seem to depend much on $z$, I get only four different possible outputs which are $0$, $0.005$, $0.035$ and $0.102$.

Two examples copy-pasted from Moodle:

1-(1+sqrt(1-exp(-0.619495805*((40.14-11.7)/4.3)^2)))/2 = 0.035

1-(1+sqrt(1-exp(-0.619495805*((33.18-13)/4.1)^2)))/2 = 0.102

Where here $\left\{z\right\}$ was actually calculated via $$\left\{z\right\}=\frac{\left\{x\right\}-\left\{m\right\}}{\left\{s\right\}}$$

Does anyone know how to work around this and get actual proper values for the normal inverse function?

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    $\begingroup$ I've never used Moodle, but according to moodle.org you cannot us ^ for powers; instead use pow({foo}, 2). $\endgroup$ – user1027 Nov 28 '19 at 21:00
  • $\begingroup$ @user1527 Oh, good point. Now I wonder how it can calculate something in some cases then. Thanks, I'll try fixing that! $\endgroup$ – Arnaud Mortier Nov 28 '19 at 22:07
  • $\begingroup$ @user1527 Well, you can post this as an answer and I will accept it. It fixed everything. I was really thrown off by the few numerical outputs, which got me thinking that it was a problem of too violent an approximation being done at some point by the software. $\endgroup$ – Arnaud Mortier Nov 28 '19 at 22:19
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I've never used Moodle, but according to moodle.org you cannot use ^ for powers; instead use pow({foo}, 2).

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  • $\begingroup$ Thanks a million! I wonder if we will ever get an explanation why it still computed something in some cases. $\endgroup$ – Arnaud Mortier Nov 28 '19 at 22:26

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