Vital horoscope: Longitudinal data collection in the Iranian primary health care system

Despite their importance, longitudinal methods of data collection are complex, time-consuming and expensive. With longer periods of observation, the problems of population movement and loss to follow-up become important. This is particularly the case in developing countries with high rates of rural-to-urban migration and residential mobility within major urban centres. A more practical alternative involves repeated data collection on well-defined communities on a regular monthly or annual basis. Combining such data-gathering exercises with a practical programme of intervention or service delivery can justify the expense involved and ensure the viability of the project.