TI-84 Plus CE, HP 15C, and HP 12C: Decoding the Gradematic 100

TI-84 Plus CE, HP 15C, and HP 12C: Decoding the Gradematic 100

GPA as a function of grade

Last December, we have gave a spotlight on the Calculated Industries 100 from 1983. The Gradematic 100 was a specialty calculator that determines the GPA average for a student or a bunch of students in a class. We can either use numerical grades, where the maximum total score and the minimum passing grade (better than E (or F)) are set, or letter grades, where the letters are given an approximated. To see the review, click the link below:

https://edspi31415.blogspot.com/2025/12/spotlight-calculated-industries.html

Using the standard grade scale for a single assignment, with a perfect score being 100 and the minimum passing grade is 60, the following scores are given the GPA:

GRADE (0 – 100)	GPA	Letter Grade Given by Gradematic 100
0	0.00	E (can stand for F)
10	0.08	E (can stand for F)
20	0.16	E (can stand for F)
30	0.25	E (can stand for F)
40	0.33	E (can stand for F)
50	0.41	E (can stand for F)
55	0.45	E (can stand for F)
60	0.50	D-
65	1.00	D
70	1.50	C-
75	2.00	C
80	2.50	B-
85	3.00	B
90	3.50	A-
95	4.00	A
100	4.50	A+

Plot of values (using a TI-84 Plus CE):

As we can see, the plot consists of two line segments: one where grades value from 0 to 60, and one where grades value from 60 and higher. It is apparent that that the two parts makes a piece-wise function consisting of two lines.

Note: the graphs and statistics were done with the TI-84 Plus CE Python (will work with any TI-84 CE family). The piecewise function is from the math-math menu.

The Gradematic 100 distributes the GPA as:

E: 0.00 (or F)	D+: 1.33	C+: 2.33	B+: 3.33	A+: 4.33
D-: 0.66	C-: 1.66	B-: 2.66	A-: 3.66
D: 1.00	C: 2.00	B: 3.00	A: 4.00

Note: The distributed GPA scales will vary among the school districts and systems. However, we will assume the system that matches the default 60/100 system.

HP 15C and HP 12C: Find the GPA given numeric grade

HP 15C Code:

LBL C	001	42, 21, 13
6	002	6
0	003	0
x≤y	004	43, 10
GTO 1	005	22, 1
2	006	2
×	007	20
÷	008	10
RTN	009	43, 32
LBL 1	010	42, 21, 1
CL x	011	43, 35
1	012	1
0	013	0
÷	014	10
5	015	5
.	016	48
5	017	5
-	018	30
RTN	019	43, 32

HP 12C Code:

6	01	6
0	02	0
x≤y	03	43, 34
GTO 09	04	43, 33, 09
2	05	2
×	06	20
÷	07	10
GTO 00	08	43, 33, 00
CL x	09	35
1	10	1
0	11	0
÷	12	10
5	13	5
.	14	48
5	15	5
-	16	30
GTO 00	17	43, 33, 00

Eddie

All original content copyright, © 2011-2026. Edward Shore. Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited. This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.

Spotlight: Calculated Industries Gradematic 100

Spotlight: Calculated Industries Gradematic 100

Quick Facts

Model: Gradematic 100

Company: Calculated Industries

Timeline: 1983

Type: Grades conversion, Four Function

Power: 2 LR44/AR76 batteries

Number of Functions: 576 (see source)

Calculated Industries has three sequels to the Gradematic 100: Gradematic 200, Gradematic 3000, and Gradematic 4000.

Get that Grade!

The Gradematic 100 has four main modes:

* Four Function Calculator

* Timer

* Numerical Grades

* Letter Grades

Four Function Calculator (Calculate Mode)

The four function calculator is the basic calculator with the four arithmetic functions (+, -, ×, and ÷), the percent function (%), and one memory register. The [ MRC ] key acts as both a memory recall and clear. However, there is no square root or change sign key.

From any other mode, we can enter the Calculate mode by pressing the equals key [ = ].

Timer Mode

The timer has the ability to count down from a set time or count up (stopwatch).

Count Down: [ TIMER ], enter minute and seconds, then press [ TIMER ] again. The Gradematic 100 does not beep when the timer runs out.

Count Up: With the display at 0 minutes and 0 seconds, press [ TIMER ] starts the stopwatch. Press [ TIMER ] again to stop the time. The stopwatch can only be stopped once.

Numerical Grades Mode

In the Numerical Grades mode, we can enter numerical grades for students depending on the total number of assignments and tests. The general process is this:

1. Press the [ NUMERICAL GRADES ] key.

2. Enter the maximum total score, which would get the student an A+ grade, then press [ STORE*HI ].

3. Enter the minimum total score of which a student to earn a D- grade, then press [ STORE*LO ].

4. To see the break down, press the [ BREAK POINT ] key repeatedly. When ready, press the [ CONTINUE ] key.

5. For each student, enter each grade, then press the plus button [ + ]. When finished, press the [ STUDENT AVERAGE ] key get the average and the grade.

6. (Optional) Repeat step 5 to enter additional students. Press the [ MRC ] key to get the Class Average.

Example: In a class of six assignments worth 100 points each (for a total of 600 points), find a students’ average on the grades: 88, 78, 85, 92, 93, 90. The minimum grade for a D- is 60% of the maximum (360 points).

[ NUMERICAL GRADES ]

Display: E Hi Sc. 600 [ STORE*HI ]

Display: E Lo Sc. 360 [ STORE*LO]

Display: b.P. Press [ BREAK POINT ]:

360 d-

380 d

400 d+

420 C-

440 C

460 C+

480 b-

500 b

520 b+

540 A-

560 A

580 A+ (press [ BREAK POINT ] to repeat the cycle)

[ CONTINUE ] Display: Su Sd Gr

88 [ + ] 78 [ + ] 85 [ + ] 92 [ + ] 93 [ + ] 90 [ + ] Display: 6 526 (total points: 526)

[ STUDENT AVERAGE ] 6 3.26 b+

Note: For some reason, the grade below D is labeled E.

Letter Grades Mode

The Letter Grades mode operates in a similar way to the Numerical Grades Mode. The Letter Grades Mode has specialty keys [ A+ ] through [ E ]. The student average and class averages works the same way.

Example: Find a students average consisting of the grades A-, B, C+, B+, A-, A.

[ LETTER GRADES ]

Display: Ent Gr. [ A- ] [ B ] [ C+ ] [ B+ ] [ A- ] [ A ] [ STUDENT AVERAGE ]

Display: 6 3.33 b+ (average 3.33 B+)

Two Scales

The GradeMatic 100 has two GPA scales: one where A is worth 4 GPA points, and A is worth 12 GPA points. This scales can be toggled with [ A=12 ] key, which can be pressed at any time.

Final Thoughts

If you get a Gradematic 100, make sure that a manual is included. I purchased one from a local thrift mart for a $1.00 but there was no instructions. That led me to purchase another Gradematic 100 on eBay.

Another fine specialized calculator that was once made by Calculated Industries.

Eddie

All original content copyright, © 2011-2025. Edward Shore. Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited. This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.

Spotlight: Calculated Industries QR Calc

Spotlight: Calculated Industries QR Calc <Title>

Quick Facts

Model: 3375

Name: QR Calc

Company: Calculated Industries

Timeline: 1993

Type: Quality Control, Statistics

Operating System: Algebraic

Digits: 7

Memory: 1 general purpose memory register plus specific variable registers 

Power: 1 CR-2032 battery

When I purchased the QR Calc, there was no manual with it. Normally, it wouldn’t present any issues because I can often find manuals online. That is not the case with the QR Calc. If there is any manual online, please email me. I’ll have to be more careful next time.

I apologize that I will not be able to describe all the functions but hopefully I describe enough features to give the reader a general idea. Please check out the sources below.

Features

Let’s start with the mathematical functions included: powers and roots (y^x, x². √), natural logarithm (ln), exponential function (e^x), reciprocal (1/x), arithmetic (+, -, ×, ÷), and the standard percent key which operates they people expect it to (%). The order of operations is enforced. Like a lot of calculators, the change sign (+/-) key is a shifted function.

The operating range of the calculator is 7 digits: -9,999,999 to 9,999,999. Any calculator that has a result outside of this range causes the QR calculator to display an error message.

The store key for the QR Calc is labeled [ Set ].

The function →PPM changes any number into n parts per million.

Example: 0.0014 →PPM displays 1,400 PPM.

Here are some functions I was able to find out and figure out:

Normal Distribution

The normal distribution functions calculate areas of the standard normal distribution, assuming that μ = 0 and σ = 1.

n [ nZ ]: lower tail probability (from -∞ to x = n)

n [ 2^nd ] (ModZ): probability from x = 0 to x = n

One Variable Statistics

The QR handles a single set of statistics with the following key and key sequences:

[ Add ]: Add a data point (Σ+)

[ 2^nd ] [ Add ] (Subtract): Subtract a data point (Σ-)

[ 2^nd ] [ + ] (S←→P): Toggle between standard deviation and population deviation (σn indicator)

[ x-bar ]: Calculate the arithmetic mean.

[ R ]: Calculate the range of the data.

[ 2^nd ] [ x-bar ] (N): Calculate the number of data points.

[ 2^nd ] [ R ] (σ): Calculate the deviation, depending on the deviation mode set.

[ Low ]: Returns the minimum value of the data set entered.

[ 2^nd ] [ Low ] (High): Returns the maximum value of the date set entered.

[ Skew ]: Skew. I’m not sure what formula the QR Calc uses because I have not been able to match results with any formula I found yet.

[ 2^nd ] [ Skew ] (Kurt): Excess Kurtosis. Kurtosis measures how concentrated the data is with respect to the mean.

The formula used for Excess Kurtosis:

μ4 = 1/n * Σ((xi – mean)^4)

kurtosis = μ4/s^4 – 3

Process Capability Indices

If a process is mature, that is a process that is regular and has been executed for a period of time, we can measure the capability index.

There are two capability indices:

Cp: for a centered analysis

Cpk: for a non-centered analysis. This metric is used for potential future performance.

Generally, we want these indices to be at least 1. Capable processes have an Cp index of 1.33 or higher.

Key strokes:

Enter the mean: [ Set ] [ x-bar ]

Enter the deviation: [ Set ] [ 2^nd ] [ R ] ( σ )

Enter the lower specification limit (based on the normal distribution): [ Set ] [ LSL ]

Enter the upper specification limit: [ Set ] [ 2^nd ] [ LSL ] (USL)

Each variable entered will have an indicator.

Calculate Cp: [ 2^nd ] [ Cpk ] (Cp)

Calculate Cpk: [ Cpk ]

Example:

LSL = - 1, USL = 1, mean = 0.05, deviation = 0.27

[ 2^nd ] [ × ] (AC) to clear out the registers if needed

0.5 [ Set ] [ x-bar ] (x-bar indicator is on)

0.27 [ Set ] [ 2^nd ] [ R ] ( σ ) (σ indicator is on)

1 [ 2^nd ] [ - ] (+/-) [ Set ] [ LSL ] (L indicator is on)

1 [ Set ] [ 2^nd ] [ LSL ] (USL) (U indicator is on)

Results:

Cp: 1.234568

Cpk: 1.17284

That is a pretty good process.

Formulas Used:

Cp = (USL -LSL) / (6 * σ)

Cpx = min((x-bar – LSL) / (3 * σ), (USL – x-bar) / (3 * σ))

Control Charts

We get to the main feature of the QR Calc: Control Charts and Capability Limits. On the back of the calculator, the QR Calc has a list of handy formulas.

The heart of the QR Calc is the table of constants that are used in control charts and limit charts. Often the chart limits are built on many samples of n data points each, where x-bar is the average of the sample averages, and R is the average of the range samples. We can also build chart limits with one sample. The QR Calc can only handle sample sizes from 3 to 25 data points.

Mean Control Chart Limits:

Lower: LCL-mean = x-bar – n * A2 * R

Upper: UCL-mean = x-bar + n * A2 * R

R Control Chart Limits:

Lower: LCL-R = n * D3 * R

Upper: UCL-R = n * D4 * R

A2, D3, and D4 are constants used in calculating control chart limits. Accessing these constants takes one argument, which is the sample size.

The A2 constant for a sample size of 3: 3 [ nA2 ] returns 1.023.

Below is a short table of constants, as determined by the QR Calc.

Sample Size n	Constant A2	Constant D3	Constant D4
5	0.577	0	2.114
10	0.308	0.223	1.777
15	0.223	0.347	1.653
20	0.18	0.415	1.585
25	0.153	0.459	1.541

Standard deviation can be estimated by using the average range ( R ) and another constant d2:

σ ≈ R / d2

Sample Size n	Constant d2
5	2.326
10	3.078
15	3.472
20	3.735
25	3.931

A table of constants from n = 2 to 25 can be found here:

https://sixsigmastudyguide.com/x-bar-r-control-charts/

Example:

Construct mean and range charts from a sample (n = 5):

3.995

4.26

4.37

4.44

4.58

Keystrokes:

(after clearing data)

3.995 [ Add ] 4.26 [ Add ] 4.37 [ Add ] 4.44 [ Add ] 4.58 [ Add ]

X-bar chart:

LCL: [ x-bar ] - 5 [ nA2 ] [ × ] [ R ] [ = ] Result: 3.991455

UCL: [ x-bar ] + 5 [ nA2 ] [ × ] [ R ] [ = ] Result: 4.666545

R chart:

LCL: 5 [ 2^nd ] [ nD4 ] (nD3) [ × ] [ R ] [ = ] Result: 0

UCL: 5 [ nD4 ] [ × ] [ R ] [ = ] Result: 12.36669

The QC calc has contains the E2 constant.

Final Thoughts

The functions that I still do not know about or have figured out are: TRGa, TRGb, RS a, RS b, %Low, and %High.

This review is incomplete. I will keep searching for a manual, I may have to buy another QR Calc.

This calculator is a rarity, and one worth checking out.

Sources

Hessing, Ted. “Process Capability (Cp & Cpk)” 6σSTUDYGUIDE.COM (no specific date give, first comment on November 19, 2014) https://sixsigmastudyguide.com/process-capability-cp-cpk/. Accessed January 2025.

Hessing, Ted. “X Bar R Control Charts” 6σSTUDYGUIDE.COM (no specific date give, first comment on April 17, 2018) https://sixsigmastudyguide.com/x-bar-r-control-charts/. Accessed January 2025.

Hewlett Packard. HP-65 Stat Pac 2 Cupertino, CA. https://literature.hpcalc.org/items/975 1975

Next time, I’m going to see if a manual comes with it. Not everything has a manual online.

Eddie

All original content copyright, © 2011-2025. Edward Shore. Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited. This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.