Читать книгу Statistics in Nutrition and Dietetics - Michael Nelson - Страница 19

Case studies provide a single example of what is believed to be true. The example is so compelling by itself that it is used to infer that the particular instance described may be generally true. For example:

A dietitian treating a severely underweight teenage girl worked with a psychotherapist and the girl’s family to create an understanding of both the physiological and psychological basis and consequences of the disordered eating, resulting in a return to normal weight within six months. The approach contained unique elements not previously combined, and could be expected to have widespread benefit for similar patients.

A case study can be interesting and provide a powerful example. But it provides very limited evidence of the general truth of the observation.

Descriptive studies bring together evidence from a number of related observations that demonstrate repeatability in the evidence. For example:

In four old peoples' homes, improved dining environments using baffles to reduce noise interference and allowing more time for staff to take orders and serve meals resulted in improved nutritional status among residents after one year.

This type of cross‐sectional evidence from numerous homes is better than evidence from a single home or a case study.

The generalizable conclusion, however, depends on a number of factors that might also need to be taken into account: what was the turnover among residents – did new residents have better nutritional status when they arrived, were they younger with better appetites, did they have better hearing so that they could understand more clearly what options were available on the menu for that day, etc.? One of the difficulties with descriptive studies is that we may not always be comparing like with like. We would have to collect information to demonstrate that apart from differences in noise levels and serving times, there were no other differences which could account for the change in nutritional status. We would also want to know if the circumstances in the four selected care homes were generalizable to other care homes with a similar population of residents.

Experimental studies are designed to assess the effect of a particular influence (exposure) on a particular outcome. Other variables which might affect the outcome are assumed to be held constant (or as constant as possible) during the period of evaluation.

Establish if a liquid iron preparation is effective in treating anaemia.

If an influence produces consistent effects in a chosen group of subjects, we are tempted to conclude that the same influences would have similar effects in all subjects with similar characteristics. When we evaluated the results from our observations, we would try to ensure that other factors which might affect the outcome (age, sex, dietary iron intake, dietary inhibitors of iron absorption, etc.) were taken into account.