June 2010 Edition
Computer Science Department, University of Cape Town
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Hierarchical Task Analysis has been in use for a long time – since the 1960’s or even earlier. It is most suitable for analysing tasks that have a well-defined structure – that is, tasks which tend to be performed in similar ways every time, rather than those that have a very loose structure. HTA involves describing the task in terms of a task-subtask hierarchy and a set of plans that define in what order subtasks may be performed, or under what circumstances particular subtasks are performed at all.
We will start with the simple example introduced earlier: writing a letter and preparing it for posting. Initially we consider the description of Linden's activity:
| Description of task | Subtasks |
|---|---|
| First, we observe Linden, who has no computer support, performing this task in her office. She finds a sheet of paper, a matching envelope and a pen, and settles down at her desk to write. | Prepare for writing. |
| She starts by putting her own address in the top right-hand corner, then picks up her address book (which is lying on the desk beside her) and flicks to the page containing the addressee’s address information. She copies the addressee’s address to the paper (left-hand side), then writes today’s date and starts on the body of the text: “Dear …”. When she has finished, she signs the letter | Write letter |
| & then reaches for the envelope, writes the name on it and copies the address from the address book onto the envelope, & | Prepare envelope |
| & folds the letter, puts it into the envelope and seals the envelope. Task completed. | Put letter in envelope |
One of the standard ways of presenting a HTA is as a tree structure:
 |
There may be some other notations you are familiar with where the order of appearance of boxes in the tree indicates ordering (probably left to right). This is not the case for HTA: the only thing that matters is the hierarchy. As well as presenting the hierarchy, it is necessary to describe plans that define the possible ordering of activities. In this case, a suitable plan would be:
Plan 0: Do 1, then 2 and 3 in either order, then 4.
Although tree structures are visually appealing – well, more appealing than the alternatives, anyway – they can be tedious to draw without a suitable tool. Therefore an alternative text-based notation that relies on indentation is often used. We will use this textual notation to expand the task description for the letter-writing task.
0: Write letter and prepare for posting
1: Prepare for writing
1.1: Get paper
1.2: Get envelope
1.3: Get pen
1.4: Get address book (not explicitly stated, but clearly necessary)
2: Write letter
2.1: Write own address
2.2: Write addressee's address
2.3: Write date and "Dear..."
2.4: Write body text of letter
2.5: Sign off
3: Prepare envelope
3.1: Write name on envelope
3.2: Write address on envelope
4: Put letter in envelope
4.1: Fold letter
4.2: Place letter into envelope
4.3: Seal envelope
Again, we need plans to describe how to perform each subtask:
Plan 1: Do 1.1, 1.2, 1.3 and 1.4 in any order
Plan 2: Do 2.1 then 2.2 then 2.3 then 2.4 then 2.5
Plan 3: Do 3.1 then 3.2
Plan 4: Do 4.1 then 4.2 then 4.3.
Task analysis involves generating as general a description as possible. So, for example, we might want to generalise tasks 2.1, 2.2 and 2.3 to a new task: write head of letter. Similarly, we might notice that it is not necessary to have the envelope to hand until the time when it is to be prepare, or the paper to hand until the point where the user starts writing the letter, but we need the pen and address book for both, so we might break down task 1. If we do these things, we get a new structure:
0: Write letter and prepare for posting
1: Get paper
2: Get envelope
3: Prepare for writing
3.1: Get pen
3.2: Get address book
4: Write letter
4.1: Write head of letter
4.1.1: Write own address
4.1.2: Write addressee’s address
4.1.3: Write date and “Dear…”
4.2: Write body text of letter
4.3: Sign off
5: Prepare envelope
6: Put letter in envelope
Again, we need plans to describe how to perform each subtask:
Plan 0: Do 1, 2, 3, 4 and 5, then 6. 3 must be done before 4 and 5; 1 must be done before 4; 2 must be done before 5.
Plan 3: Do 3.1 and 3.2 in either order
Plan 4: Do 4.1 then 4.2 then 4.3.
Plan 4.1: Do 4.1.1 then 4.1.2 then 4.1.3
Plan 5: Do 5.1 then 5.2
Plan 6: Do 6.1 then 6.2 then 6.3.
We see that now different parts of the task analysis are presented at different levels of detail. This is often thought of as a Bad Thing, but in this case it allows us to describe the optionally and alternative orderings within the task more clearly. As with most aspects of design, there is no perfect solution just solutions that are better or worse for particular purposes.
Look at the paragraph that describes Remi’s word-processing task (reproduced below). Mark up this paragraph into activities that sensibly constitute a subtask, using the analysis already presented for Linden’s activity as a guide. Construct a HTA description of this task.
We then go to the next office to observe Remi using his word processor to do something similar. He settles down in front of his computer and opens his word processing package, using a document template that already includes his own address and today’s date. He opens his electronic address book, finds the appropriate address, and uses his mouse to copy and paste the address from the address book into his document. He starts on the body of the text: “Dear …”. When he has finished composing the letter on the screen, he selects the ‘print’ option and waits until a message appears on his screen to say “Your document Letter to KME has been printed on Laser 6”. He gets up and walks out to the shared printer area, heads for the printer in the far corner, and flicks through the pages that have accumulated there until he finds his letter. He picks it up, walks back to his desk, reads it through and signs it. The printer does not print on envelopes, so he finds a sheet of mailing labels that will fit through the printer. He has a document template that is set up to print in the format appropriate for mailing labels, so he opens that and copies the information from his electronic address book to this new file. He selects the print option, remembering to select the option for ‘manual feed’ (so that he can place the blank mailing label sheet in the manual feed tray of the printer). He then walks out to the printer and inserts the mailing labels in the manual feed tray then hurries back to his office to click on the “OK” button that has now appeared next to the message “Manual feed: click OK when ready to print”. He walks back to the printer again to collect the printed label, then goes to find an envelope. He folds the letter and slips it into the envelope, sticks the mailing label on the envelope, then seals it. Task completed.
Using the guidelines for adaptation, and the example of modifications presented above, modify your HTA so that it better matches the general task of using a word processor such as that being used by Remi
Answer at the end of the chapter.
You should now have HTAs for two similar tasks: writing a letter by hand and word-processing a letter. Hopefully, they share many of the high-level tasks in common, and differ mostly in the detail. Use these HTAs as a starting point for adapting the computer-based task so that it is easier for users.
Answer at the end of the chapter.
Construct a HTA for finding information on the Word Wide Web, starting with data gathering. If possible, you should observe a few other people (fellow students or friends) performing the task, and note down everything they do. If this is not possible, perform the tasks yourself, and note down everything that you do. Use this data as a basis for constructing a HTA. The information seeking should cover two different kinds of information seeking tasks, as described here.
There is a shareware text on Task Analysis, written by Clayton Lewis and John Rieman, available on the web. Use the Google search engine (http://www.google.com) to start the search, and observe how people behave from there. When the text has been found, you might like to read some of it!
Imagine you are planning a trip to Sydney (Australia) or Boston (USA). You need to identify a cheap flight and also find a suitable hotel to stay in. Get all the information that you would need to be able to just book over the Web or telephone to make a reservation.
Use the HTA produced as a starting point for thinking about how the searching task can be made as easy as possible for users. Note that the design of search engines is a separate topic that is beyond the scope of this course!
A Discussion on this activity can be found at the end of the chapter.
One of the hardest things to grasp about HTA is how to construct a ‘good’ one. In one respect, HTA is very easy: do some hierarchies and some plans. In another, that superficial simplicity hides a deeper challenge: what makes a ‘good’ HTA is its fitness for purpose, and assessing that is usually difficult. For this reason, it is important to construct HTA descriptions, and then to modify and rework them, thinking about alternative structures, and about what level of detail it is most useful to go to.
As noted above, HTA differs from GOMS in that it considers only physical activity (not cognitive tasks), and focuses on plans (rather than selection rules). Conversely, large-scale tasks can be described using HTA whereas only a dedicated person with too much times on their hands would complete a GOMS analysis for such large tasks (it would take weeks of effort to do it thoroughly). If using HTA as a guide for implementation or for detailed documentation, a fairly fine grain of detail (down to the individual action level) may be necessary; where it is being used to help get an understanding of the domain tasks, less detail may be appropriate.
A HTA description consists of two main components. What are they?
Answer at the end of the chapter.
HTA is a type of Cognitive Task Analysis. True or false?
Answer at the end of the chapter.
Describe the process of constructing a HTA description.
Answer at the end of the chapter.
Conduct a HTA for an imaginary fast-food delivery outlet (e.g. pizza delivery), from the point where the customer rings in to place an order to the point where the person who has made the delivery returns with the payment.
Answer at the end of the chapter.