To be collected in a similar fashion from both the groups regarding their exposure status and vulnerability to the disease or expected outcome
This base-line information serves as a benchmark for future analysis and other considerations.
Sources of information vary according to the availability and feasibility. They may be records and registers, medical examinations and investigations, interviews and environmental influence or contributions.
Time, duration and extent of collection of data not only determine the design of cohort study but also influence the validity of the study.