One of my tasks as pro vice-chancellor is to ensure that University of Portsmouth students and staff know when they will meet: timetabling - surprisingly complicated and bizarrely provocative.

The first problem is that numbers are uncertain until September. We cannot delay timetabling until clearing, so we use "best guesses" for class size and allow a safety margin. Even so, we can be caught out, as happened in 2008 when our intake was large. Variations are less likely now that numbers are capped, but subject variations still do happen.

We decided that we needed a reliable timetable much earlier than August. Many students have families and need to make childcare arrangements, and an erratic timetable that appears as teaching begins is unacceptable.

We have a timetabling process that sets out a schedule for confirming course structures, option choices and resource requirements. On investigation I found nothing fundamentally wrong with it: the problem was behaviour. If everyone followed the rules, we would have a reliable timetable in good time. However, long after the deadline individuals were pressing the timetabling office to make changes. In effect, the published timetable was being treated as the start of a negotiation. The timetablers (lovely people) almost never said "no" and made huge efforts to accommodate every request. This often changed the whole timetable. Just like the proverbial butterfly flapping its wings, what appeared to be a small change ("can I move my lecture to Thursday?") often had extensive consequences.

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Some staff applied extra constraints, including the likes of "I want all my teaching to be on three consecutive days" and "I don't want to teach before 10am because my students won't turn up". The solution to this was to stress to the timetablers that they should say "no", and refer dissenters to me.

Having a "departmental identity" is important and we make every effort to timetable students in or as close as possible to their school. A certain amount of movement is inevitable. The university is pretty full (or, as a space consultant put it: "Portsmouth's space utilisation is so good that it compares favourably with the best in the sector."). If we had more space we would have fewer problems, but space costs money.

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Another problem results from too much choice. Some parts of the university offer options to students in their second and, in some cases, first year of study. From a pedagogic viewpoint, I do wonder how able first-year students are to make option choices.

Having lots of options is good, but it does create many classes. I also found well-intentioned individuals urging their students to "sample as many options as you like and tell us your final choice by Christmas". The consequences of this were dire. Seminars that were expected to attract 20 students, and that were timetabled in a room that held 30, suddenly grew to 40 or more when "visitors" appeared. Students complained that they were forced to sit in the gangways, which is never a good experience, quite apart from the risk if the fire alarms go off. We have taken steps to reduce the number and level of options on offer and to limit "option tourism".

The agreed calendar for making changes should mean course structures are fixed well before timetabling begins. However, it was not unknown for schools to undertake summer restructuring, and cause complete chaos. One school even decided to change its course in mid-October (you know who you are). The problem has, I hope, been resolved by deans taking a more managerial approach.

There is a tension between activities designed to enhance the student experience and the need to create a workable timetable. We currently offer a wide selection of "elective units" designed to help students acquire extra skills. We are reluctantly reducing the range of our elective provision. We simply cannot timetable elective units taken by a small proportion of students. A reduction in the freedom of a few is, sadly, necessary to protect the interests of the majority.

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The most important conclusion I have come to is that we need to be disciplined about how we structure and change courses. I have been told many times that "the ability to change how I deliver my course is my academic freedom". My response is that ad hoc changes damage the student experience, and that to safeguard the interests of the majority, the freedom of individuals to make course changes must be constrained.

The other thing I have learned is the importance of empowering those responsible for creating the timetable. Timetablers must feel that they have the authority to say "no", and must know that they will be supported by the university hierarchy.

It would be interesting - and immensely helpful - to hear whether others have had similar experiences.