Stages in the collection of longitudinal data
When a subject enters the study it is referred to as entry or enrollment. The first set of measurements taken at the start of the study are usually called the baseline observations.
After the baseline there is a period of follow-up when the subject is under observation in the study. During follow up the research team will carry out at least one more set of observations on each subject. The total number of observations and the time interval between observations will vary between studies and will depend on the outcomes of interest. For example, a study of new mothers established to investigate breastfeeding is likely to run for no more than two to three years i.e. the maximum duration of breastfeeding. A study set up to measure survival post HIV infection would need to run for two to three decades.
Being under observation does not necessarily mean that there is any activity from the research team between the observations. However in studies that have long intervals between observations considerable problems may arise in maintaining contact with all the study subjects.
A subject leaves the study when a fixed time point is reached or a pre-defined outcome occurs.
Some subjects will not complete all the sets of observations but will leave the study early. These subjects are said to be lost to follow-up and are potentially a problem for studies collecting longitudinal data. If certain types of people are more likely than others not to complete the study there is potential for a bias in the results.