Statistics, calculator usage: the normal distribution on the Casio fx-991ES PLUS C

Tutoring statistics, one uses all kinds of calculators. The tutor gives an example of how to use the Casio fx-991ES PLUS C for normal cdf calculations.

The following is according to my understanding.

As calculators go, it seems to me the Casio fx-991ES PLUS C might be the only scientific calculator a person might need. As a bonus, it does some common distribution calculations, like with the normal distribution.

To use the normal cdf on the Casio fx-991ES PLUS C, one would press the Mode key, then the bottom part of the circular button at the top, where an arrow points down. Doing so reveals a menu with 3: DIST as a choice: one would key 3 at that point.

Next, another list appears, with Normal CD as choice 2: one would key 2.

Now, the familiar question “Normal CD: Lower?” appears. If the user just wants to find the proportion of the population less than a certain value, they might key in -10xσ (σ is the standard deviation), then press =. Otherwise if the user wants the proportion of the population between two values, they would enter the lower value, then press =.

Next, the prompt “Normal CD: Upper?” appears. If the user just wants the proportion of the population below a value, this is where to key that value, then press =. Otherwise, the user will enter their upper value here.

Next, the prompt appears, “Normal CD: σ?” This is where to enter the standard deviation.

Then, the prompt appears, “Normal CD: μ?” This is where to enter the population mean, aka average.

Then, p= will appear with the answer.

Example: Imagine a population of essays with mean word count 2000 and standard deviation 211 words. Calculate the percent of essays with a) word count less than 2050 and b) word count between 1700 and 1800.

For the solution to a), one would key MODE, then the “down” arrow on the top circular button, then 3, then 2. The user isn’t looking for the proportion between two values, but simply less than a value. Therefore, when it asks for “Normal CD: Lower?”, the user can key in -10xσ, which is -2110, in this case. Next, at “Normal CD: Upper?”, the user can key in 2050. At “Normal CD: σ?”, the user can key in 211, then at “Normal CD: μ?” they can key in 2000. The answer, it seems, is 0.594 rounded, or 59.4%: apparently, 59.4% of the essays will have word count less than or equal to 2050.

For the solution to b), one would key MODE, then the “down” arrow on the top circular button, then 3, then 2. At “Normal CD: Lower?”, they would key 1700. At “Normal CD: Upper?”, they would key 1800. At “Normal CD: σ?”, they would key in 211, then at “Normal CD: μ?”, they would key in 2000. The answer, it seems, is 0.094 rounded, or 9.4%: apparently, 9.4% of the essays will have word count between 1700 and 1800.

BTW: to get back to regular calculation mode, one might try MODE then 1, which, as I understand, is “computation” mode.

For background about the normal cdf, one might look to my post from January 24, 2026. For a comparison between how a spreadsheet might handle the normal cdf, as opposed to a calculator, one might see my post from January 28, 2026.

Source:

casio.com: fx-991ES PLUS C User’s Guide