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What do you call a curve that is just the first half of a bell curve. For example, let's say in a typical bell curve of letter grades, a few students get F grades most get C grades and just a few get A grades.

I'd like a curve that is the first half of the bell curve so that a few students get F grades and the most common grade is an A grade.

(I'm not actually doing this for grading. It's just the example that came to mind.)

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    $\begingroup$ what kind of answer you are expecting? The first half of a bell curve is the first half of the bell curve. Even if it had specific name, I do not understand how would that might help. $\endgroup$
    – mpiktas
    Commented Mar 21, 2011 at 8:18
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    $\begingroup$ if we take first half of a bell curve, then we truncate the original distribution. So truncated normal distribution might be something you are after. $\endgroup$
    – mpiktas
    Commented Mar 21, 2011 at 8:23
  • $\begingroup$ The shape you describe can be variously described as 'sigmoid'. Is this what you are after? It is quite unclear what this question is about (specially because the sample is a discrete distribution with three values only, which can be equally called triangular instead of bell-shaped). $\endgroup$
    – Firebug
    Commented Aug 27, 2023 at 16:33

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A "bell curve" in the non-technical sense could refer to one of a family of statistical distributions which are bell-shaped. In the context of grading I've only ever seen the normal distribution (and it is by far the most common in general), but others include the logistic, t, etc. The half-normal distribution is generated by taking the absolute value of a (zero-mean) normal distribution. That is to say, it represents the case where most students get an F and few get A's. By making a suitable transformation of this distribution (taking the negative to get the mirror image then shifting so that the maximum is 100 points), you can get the distribution you're after.

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