1.Retrospective studies have the disadvantage that they rely on data already recorded (such as hospital records) or on people remembering being exposed to a potential risk factor. Hospital records are likely to have recorded salt intake in more detail when the patient visited because of hypertension than if their visit was due to an unrelated disease, for example having a hernia. Information remembered by patients might be subject to recall bias; for example, people who experience a stroke would tend to remember their salt intake in the last months or years in more detail than those that did not suffer a stroke. However, as the outcome (hypertension, for example) has already occurred when the subjects are selected, the studies are less time-consuming to conduct.
2.Prospective studies are based on collecting information at a given point in time, and then revisiting the subjects at a later date. The type of information that could be collected in such a study could be, for example, urinary sodium excretion and a given outcome, such as stroke. The advantages are that you focus in more detail on specific measures and outcomes. Much greater attention can be given to the sample of subjects that are selected, and to the study design. The potential weakness is unanticipated loss of a proportion of selected subjects in the follow-up period. The study by Tuomilehto et al. (2001) is an example.
3.The main limitation of a cross-sectional study is that it is often difficult to determine whether the risk factor led to the outcome. However, it will give the prevalence of a certain outcome (at a given point in time) and the possible risk factors. For example, if a survey was carried out and hypertension prevalence was found to be positively correlated with salt intake, it would be difficult to know if the hypertension was caused by salt intake.
4.Intervention studies have the advantage that the researcher knows which individuals have been exposed and which ones have not. An example is the study by Mei et al. (2010), in which diets containing different amounts of sodium and potassium were given to selected families, and their relationship with blood pressure was evaluated. However, ethical concerns can arise depending on the intervention and the study design.