## Question types: STACK

### Sets

The STACK system is a computer aided assessment package for mathematics, which provides a question type for the Moodle quiz. In computer aided assessment (CAA), there are two classes of question types.

**Selected response questions**

In these questions, a student makes a selection from, or interacts with, potential answers which the teacher has selected. Examples include multiple choice, multiple response and so on.**Student-provided answer question**

In these questions the student's answer contains the content. It is not a selection. Examples of these are numeric questions.

STACK concentrates on student-provided answers which are mathematical expressions. For example, a student might respond to a question with a polynomial or matrix. Essentially STACK asks for mathematical expressions and evaluates these using computer algebra. The prototype test is the following pseudo-code.

```
if
simplify(student_answer-teacher_answer) = 0
then
mark = 1,
else
mark = 0.
```

STACK uses a *computer algebra system* (CAS) to implement these mathematical functions. A CAS provides a library of functions with which to manipulate students' answers and generate outcomes such as providing feedback. Establishing algebraic equivalence with a correct answer is only one kind of manipulation which is possible.

Using CAS can also help generate random yet structured problems, and corresponding worked solutions.

In STACK a lot of attention has been paid to allowing teachers to author and manage their own questions. The following are the key features.

- Question versions are randomly generated within structured templates.
- There are many different kinds of inputs. These are, for example, where the student enters a mathematical expression, or makes a true/false selection.
- Mathematical properties of students' answers are established using answer tests within the CAS Maxima.
- Feedback is assigned on the basis of these properties using a potential response tree. This feedback includes:
- Textual comments for the student.
- A numerical mark.
- Answer notes from which statistics for the teacher are compiled.

These broadly correspond to formative, summative and evaluative functions of assessment. Which of these outcomes is available to the student, and when, is under the control of the teacher.

- Multi-part mathematical questions are possible: each question may have any number of inputs and any number of potential response trees. There need not be a one-to-one correspondence between these.
- Partial credit is possible when an expression only satisfies some of the required properties.
- Plots can be dynamically generated and included within any part of the question, including feedback in the form of a plot of the student's expression.

I must confess I am only a veterinary pathologist who enjoys translating Moodle into spanish, and it's been 30 years since I took calculus, but one English language string in qtype_stack seemed odd to me:

2.4 [ddl_empty,qtype_stack] "No choices were provided for this drop-down. Please input a set of values link a,b,c,d"

I wonder if the word "link" should not really be "like".

I apologize if am asking a silly question and I thank you in advance for your help.

In fact, the drop-down list input type is not yet working in STACK 3.0, so this string is not used anywhere, so the typo does no harm.

I tried to instal the plugin several times. I think I followed step by steps the instructions but when running STACK healthcheck I get the message:

The version of the STACK-Maxima libraries being used (So old the version is unknown!) does not match what is expected (2012122800) by this version the the STACK question type. It is not really clear how that happened. You will need to debug this problem yourself.

And CAS result

Warning, empty result!

unpack_raw_result: no results were returned by the CAS.

Unpacked result as

Array

(

)

Has anybody similar problema?

It looks to me like PHP could not start a CAS process at all. Hence, there is no version number being returned. If you email me the HTML page you get back from the healthcheck, and let me know a little about your system I will be able to help more.

If you are on unix you need to ensure maxima is installed and working. Try the command line "maxima".

Chris

Getting this error with latest release:

Coding error detected, it must be fixed by a programmer: Cannot require a CSS file after has been printed.

Was able to test with Moodle 2.5 and previous release. Any idea what to verify?

Thanks

Can you try again with Debugging set to DEVELOPER level, and send us the full error message, including the stack trace.

I hope you don't mind that STACK is now listed in http://docs.moodle.org/25/en/Question_types#Third-party_question_types

and there is a Moodle Docs page at http://docs.moodle.org/25/en/STACK_question_type , whish is also translated into spanish.

The STACK plugin English language strings are 90% translated into spanish and the translation should be finished in a month.

If you are not sure whether something is a bug or not, or if you just want advice, then the Moodle Maths forum (linked to as 'Discussion' above) is the best place to ask.

Thanks in advance!

Kevin

I'm using STACK 2014032800 (installed via git). None of my plots are showing grid lines; they do show zero lines. ("The grid has been changed by adding the gnuplot commands set zeroaxis, set grid. This adds more grid lines than Maxima's default...") I am using Gnuplot 4.2.6 and Maxima 5.32.1 (I compiled Maxima on my OSX Lion machine, but not gnuplot):

STACK_SETUP(ex):=block(

MAXIMA_VERSION_NUM:31.3,

TMP_IMAGE_DIR:"/Library/WebServer/moodledata/stack/tmp/",

IMAGE_DIR:"/Library/WebServer/moodledata/stack/plots/",

PLOT_TERMINAL:"png",

PLOT_TERM_OPT:"large transparent size 450,300",

DEL_CMD:"rm",

GNUPLOT_CMD:"/Applications/Gnuplot.app/Contents/Resources/bin/gnuplot",

MAXIMA_VERSION:"5.31.3",

URL_BASE:"!ploturl!",

true)$

/* We are using an optimised lisp image with maxima and the stack libraries

pre-loaded. That is why you don't see the familiar load("stackmaxima.mac")$ here. */

Any insight as to why grid lines are not showing up? In case it's relevant, changing [nticks] has no effect, either.

Thanks again,

Kevin.