THE TIME POINTS

We need to choose the time points at which we are going to observe or measure the subjects. The more times the better will be our picture of each subjects profile, but the greater will be the cost in time, effort or money. In some experiments, taking measurements might interfere a little with what is being observed (e.g. a behaviour experiment in which blood samples are taken), and so there is further incentive for reducing the number of observations.

It is not possible to give a set of rules for choosing time points. The choice will most often be a matter of common sense and judgement. Here we simply draw attention to a number of points which need to be considered.

• It is not necessarily an ideal that the time points be uniformly spaced. It may be practical to have more frequent observations when rapid change in the variables being measured is expected.

• It is best if all subjects have the same schedule of observation times, although this can sometimes not be realised.

• Observations may be aggregated over an interval of time, rather than measured in an instant. This should cause no difficulties. The measurements may be allocated to the centre of the time interval.

• If different individuals are observed on different days, weeks etc., then between-day differences may arise. It is important that blocking and randomisation be used to guard against such differences being confused with treatment effects.

• When observations cannot be made at exactly the time scheduled because several have to collected in sequence, then randomisation between treatments should be used to guard against biases.

• Where possible, between time effects caused by, for example, different observers, should be avoided.

• The data collected can be analysed statistically before the experiment is completed, but caution is required if the results of such analysis are allowed to influence the conduct of the remainder of the experiment.

Antedependence Modelling 27.2.96