Human-Computer Interaction 3e ­ Dix, Finlay, Abowd, Beale

exercises  -  18. modelling rich interaction


Look again at the tea making task analysis in Chapter 15 (figure 15.4). Go through this and look for triggers and placeholders. You will need to make assumptions (e.g. is the kettle the kind that whistles when it boils?) so document these.

answer available for tutors only

We'll list the main task sequence first and then look at the sub-tasks for tasks (1), (3) and (5).

0. make cups of tea

triggers - the whole task may be triggered by thirst (internal event), or perhaps by guests arriving(external event).

1. boil water

triggers - the trigger for this is that you have just gone to the kitchen to make the tea.
placeholders - in the kitchen with no boiling kettle and no made tea! However, this would also be the placeholder for other tasks in the kitchen (e.g. fetching biscuits). It is likely that capture errors may occur here for less frequent kitchen tasks where you go to the kitchen and bring the wrong thing.

2. empty pot

triggers - just put kettle on and standing in kitchen
placeholders - pot filled with cold tea (yuck!) - not this was already in the plan ... the plan says do (1) first ... there is no intrinsic sequence, so it would be possible to empty the pot then put the kettle on to boil, but this would be 'inefficient', wasting time - does this matter in a domestic kitchen?

3. make pot

triggers - kettle boils ... but how do you notice? Are you observant? Standing waiting for it to boil? Perhaps it has a whistle when it boils? [[Alan says he can spend a whole morning putting the kettle on to boil, not noticing that it has boiled (missed trigger) and then restarting the process ...]]
placeholders - empty pot, kettle hot (??light off for electric kettle) ... if you forget to put the kettle on before emptying the pot, it would be possible (especially with an electric kettle, or if the kettle had already been warm) to pour cold water in the tea pot!

4. wait four or five minutes

triggers - this is not a 'regular' task, so more to remind us, in the HTA, that there is a delay. However, because it is there it makes the trigger for (5) more important as you are likely to do something else (chat, wash dishes, etc.) while waiting.
placeholders - the placeholder for this is "pot ready and cups unfilled" ... which will be the same as (5) below ... how do you know that (4) is over? Do you check the time on the kitchen clock (which is not in the HTA)? Do you set a little clockwork timer (again not in the HTA)? Do we just rely on a good internal body clock, or perhaps occasionally pour a tiny drop into a test cup to see if the tea has brewed?

5. pour tea

triggers - four or five minutes have passed - a temporal trigger, but like all such triggers: how do you know? If you are very forgetful you may set an explicit timer, or simply rely on remembering after the right sort of time. Clearly potentially unreliable.
placeholders - the full pot and empty cups tell you that this is where you are ... but not that a full cold pot is the trigger for (2). If you use a tea cosy (little woollen hat over the tea pot), you may not be able to tell easily whether it is hot or cold. ... Could a very absent minded tea maker pour yesterday's cold tea?

Subtasks of - 1. boil water

1.1 fill kettle

triggers - this is the first sub-task so the trigger is effectively the (1) trigger.
placeholders - largely mental through routine, with backup "kettle empty and no gas"

1.2 put kettle on hob

triggers - kettle is probably in your hand after filling it
placeholders - same as trigger

1.3 turn on and light gas

triggers - just put kettle down (immediate trigger) ... with backup (below) of "kettle on hob and not lit" - however, this is a fairly poor trigger as it is not very salient unless one explicitly thinks "where was I?"
placeholders - partly mental "just put kettle down" ... but if you were interrupted then the secondary placeholder would be "kettle on hob and gas not lit". However, unless you remember to pick up the kettle to check it is full, this is identical to the backup placeholder for (1.1). Then if you erroneously try to refill the kettle this is not too bad ... but if you accidentally skip task (1.1). and do (1.3) instead, a potentially empty kettle is being heated ... wait for that smell of burning metal :-(

1.4 wait for kettle to boil

triggers - like (4) this is a place where the HTA is emphasising a delay
placeholders - the gas is on and the kettle not boiling ... how obvious is the lit gas? Can you mistake this for 1.1. or 1.3 placeholder and stand waiting for a kettle that is not lit? This is perhaps easier with an electric kettle where the signs are even less obvious.

1.5 turn off gas

triggers - kettle is boiling! ... as with (3) how do you notice? If you fail to notice with a gas kettle then the steam will do an effective job of stripping your kitchen wallpaper and eventually the kettle will boil dry. Secondary trigger (see below) is a gas burning with the kettle removed.
placeholders - boiling kettle on hob ... but the boiling kettle is also a trigger for (3) ... what happens if you take the kettle off the hob to pour water into the tea pot (a good idea to make sure it is really boiling when it hits the tea leaves) and then leave the gas burning on the stove. The kettle will no longer be actively boiling; there will simply be a gas burning with nothing on it. You may notice the gas burning (secondary trigger), but with an electric ceramic hob there is no visible sign with consequent danger of touching or putting something down on the hot surface.

Subtasks of - 3. make pot

3.1 warm pot

triggers - same as (3) kettle has boiled. [[note: a really expert tea maker may do this warming using hot water from the tap or with partially boiled water to avoid the kettle cooling or overboiling before (3.3)]]
placeholders - empty cool pot

3.2 put tealeaves in pot

triggers - immediately after (3.1)
placeholders - empty warm pot with no tea in it

3.3 pour boiling water

triggers - immediately after (3.2)
placeholders - empty warm pot with tea in it ... but notice that the placeholders for (3.1) and (3.2) are not so different ... have you ever added tea twice, or poured in hot water without adding tea?

Subtasks of - 5. pour tea

5.1 put milk in cups

triggers - note that this task occurs first as the initial task and then for each task. There is thus an initial trigger for the overall task (5) and then a sense of "I'm in the middle of making tea with milk jug and tea pot in my hand"
placeholders - empty cup ... and we know which cup because it is the 'first' empty one in whatever sequence we are filling them (clockwise, left to right)

5.2 fill cup with tea

triggers - just done (5.1)
placeholders - cup with milk and no tea

5.3 do sugar

triggers - finished last cup for 5.2
placeholders - all the cups are full ... but this will be true after as well?

5.3.1 ask guest about sugar

triggers - first time round because (5.3) has been triggered, subsequently because one is "in the middle" of doing sugar ... if interrupted could you forget to ask all guests?
placeholders - some guests not asked ... need to remember which ones ... but if they are sitting there not so hard

5.3.2 add sugar to taste

triggers - just got answer "yes please, N spoons"
placeholders - you need to recall which guest ... relies on memory, the tea looks the same with or without sugar! If interrupted between 5.3.1 and 5.3.2 two errors may occur: (i) you may have formulated the intention to put the sugar in and not done it ... when you return to the tea process you may recall the intention and think you have done it! (ii) you may lose count, (iii) you may have added the sugar, but the act of putting sugar in cups is similar for each guest so hard to recall and you may erroneously add sugar twice for the guest.

Finally …

Did you realise making a cup of tea was so complicated! In practice we rely on the fact that we do most of the actions habitually, but most of the potential failure points do occur if we are distracted or interrupted in the middle of the processes. For an analyst looking at a situation these may be points for technological intervention to help prevent failures (e.g. adding a whistle to the kettle if it doesn’t have one). If we are a designer planning a technical intervention we should notice where and why the triggers and placeholders work in order to prevent us creating failure points. For example, if we install a new very silent electric kettle it may be hard to notice whether or not it has been turned on or has boiled.

Other exercises in this chapter

ex.18.1 (ans), ex.18.2 (tut), ex.18.3 (tut), ex.18.4 (tut), ex.18.5 (tut), ex.18.6 (tut)

all exercises for this chapter

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