The formalisation of retrospective studies which has been well described elsewhere, (Gibson et al) has had the advantage of increasing the quality of data collected. Even the simplest things, forcing the use of standardised data collection forms, and pre-specified hypotheses has the effect of ensuring that the data that is in the charts and is more truly represented in what is collected in the data sheet. The involvement of multiple data collectors and their training has expanded the scope of what can be gathered from charts. In particular it is expanded the number of charts that can be reviewed.Consequently the quality of retrospective studies can be expected to improve and has improved over time. Individual investigator biases should be less likely to be carried over into research conclusions.

 The disadvantage of this approach is that the rigourous pre-formulation of hypothesis creates the risk that novel observations buried in the data may be missed. It is often said that retrospective studies should be considered only hypothesis generating. (This is only partially true. It may be possible under some circumstances to use retrospective data to prove causation. The occasions when this can happen are limited but it is nonetheless possible.)Hypothesis generation involves a deep knowledge of the subject matter involved, however it also involves a certain amount of serendipity. This can occur when examining large numbers of charts of patients or otherwise reviewing large numbers of cases. This requires a certain amount of flexibility in what one is willing to consider. One approach is to insist on collecting a large number of data points and doing retrospective studies. Having once collected 119 data points per visit from paper charts the prickly cactus understands that this is not trivial undertaking. Moreover it requires extensive training if non-physician data abstractors are used.

 Such approaches are considered data dredging and frowned upon in many circles and but it is only by going on fishing trips that we can hope to find fish. Providing we are willing to recognize the limitations of these, and prospectively validate them, useful information can be discovered. This in the cactus's opinion is far preferable to collecting small datasets retrospectively and then reinforcing old knowledge, but learning nothing new.

 Prospective data collection is also prone to the same problem. In some ways prospective data collection is even more prone to it as there is a certain imperative in ensuring that the data collection forms are not unwieldy. Consequently every question is carefully thought out and the number of questions to be asked is limited. This is of course a good thing, but it risks missing new risks and potentially even causes of disease. Again the risk is the is same as in retrospect of research which is too narrowly focused. Established knowledge gets reinforced, and new knowledge waiting to be discovered remains hidden.

Ironically therefore the advent of electronic medical records, could if used correctly, make retrospective research more exciting than prospective research. At the very least clinical researchers must now be far better trained in database and data management than in the past.