This section presents those observations which, whilst agreeing with the model, also present a challenge in the form of increasing the detail of behaviour which must be addressed.
4 . 4 . 1 . F orm of the expressed schedule
It was observed that when subjects expressed their schedules, they did so in one of two ways (although this could vary within a subject - see later). Some subjects ordered their forthcoming activities with respect to clock times, i.e. absolutely. The alternative was that the ordering of tasks was expressed as relative to each other. The latter case was probably most common. For example:
SI “So the first thing is to get the oven on ... the new potatoes and the cheese sauce will take about the same time to cook. I won't even have to start doing that until the fish has gone in .... and just before I'm putting the cheese in I'll put the mangetout in. New potatoes can go on as I start making the sauce”
and later the same subject continues:
S1 ‘Tve got to rem em ber... and to turn the water up for the potatoes so its boiling
before I put the pan of water on for the m angetout,... I'm debating how long the fish will be cooked for and how long it will take the sauce, I think I should put the fish in before I start cooking the potatoes or start doing the sauce, although the potatoes will probably go on fairly soon afterwards.”_________________________ The fact that this subject, along with others, does not use an absolute schedule, does not mean that she has no conception of the time that it will take, as the following statement illustrates:
S 1 “Check what time it is - just after 20 past 4, so it should be ready at about a quarter to 5 - oh plenty of time.”_________________________________________________ A good example of a subject using a real time schedule is S5 (although S2 and S8 are
similar). This subject begins by establishing the critical path:
S5 “Erm, well, what I'm going to do first is to do the thing that's going to take the longest - which is obviously the fish”______________________________________ and then refining this:
S5 “OK right I'm going to wash the - no, first I'm going to put the cooker on”_______ at which point it starts to emerge that she intends to rely on clock time:
S5 “Cooker set at 375F and timed to go on at 5 o'clock.”_________________________ Shortly after this, she states a more comprehensive sequence of tasks on a real time basis:
S5 “Right, that's going to go in at 10, 5 past. ... If I put the fish in at 5 past 5, the meal should be ready or the fish should be ready to eat at half past, assuming it takes 25 minutes, so the potatoes, which should take 20 minutes should go on - well 15 to 20 minutes, I'll put on at about 10 past, after cutting them up. ... Right, the beans will take 5 minutes to cook, so I'll ignore them”_____________________ Later, there is evidence to suggest that she is indeed using such an absolute schedule: S5 “Right, the water for my potatoes is getting near boiling and its nearly time for the
fish to go on.”_________________________________________________________ However, it is observed that even subjects who construct their high level schedule based on clock time, can end up, later on, with a relative schedule.
unapier ^
S5 “What do I still have to do? And while the fish and the potatoes are cooking, I'm going to make the roux and just before I come to the end of making the roux, I'll put the peas on because I like them al dente.”________________________________ One can speculate on the relative advantages and disadvantages of one form over the other. For example, it is possible that an initial order of cooking based on clock times is easier to construct, since it can be done with only minimal cross referencing
between tasks. Such a schedule may then reveal the points where such cross referencing is necessary.
4 . 4 . 2 . Preserving the Schedule
It is often the case that an activity turns out to need more or less time than has been scheduled for it (in other words, the schedule was inappropriate or wrong for some reason). When this happens, there are two things that a person could do to rescue the situation. One of these is to reschedule everything else around this new time
estimate, the other is to try to preserve the existing schedule and amend the new required time to conform. The protocol data suggest that the latter is the case wherever possible. E.g.:
5 1 “the potatoes are taking longer to cook than I thought they would do, so that's going to set my timing off.”
52 “Well these potatoes are looking underdone, I'm not very happy with them at all, so I'm going to stick them back in and turn it up as high as it will go.”
S7244 “i think the potatoes are on time, but everything else is late - well no, the trout is late. ... The potatoes look excellent, and I'm going to turn them down I think at this point, because I don't want them to do too much more, so I'll turn them down” S8 “Just stick a fork in the potatoes and see how they are doing. And they're rock
hard, good. And they've got 2 minutes to cook completely. So we might have to adjust our fin a l..., well I underestimated the time it might take for them to do. I thought they might take 20 minutes, but they are actually quite big new potatoes so they could well take 25 to 30 minutes to cook through properly.... So what I'm going to do, I'm going to cut them in half now because I don't want to w a it.... this is like a quick remedy, to speed things up so that they're in time with the fish, because I don't want to overdo the fish.”
There are no examples of the alternative behaviour, which would involve the person noticing that the scheduled time was wrong and rescheduling all the activities.
^ n a p i e r
In the model, this tendency is obviously a property of the Solver, but it could be thought of in at least two different ways. It could be a by-product of a partial, opportunistic, planner. Such a planner would, by nature, tend towards localised planning rather than global scheduling, and so a plan is more likely to be amended locally in the observed fashion.
Alternatively, one could imagine that it might be desirable for a planner to have such a bias towards local modifications built into it. Such a heuristic would function to economise on the effort expended by the Solver, given the assumption that making local changes requires less effort than globally rescheduling everything. Of course, there is likely to be an element of real world constraint such that, for example, it would be nearly impossible to affect the progress of some of the other tasks, and thus the only hope is local.