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Evaluating studies: First weigh all the evidence


Table 2: Retrospective studies
In a retrospective study, all of the events of interest have already occurred, and the data is obtained from the medical records and from recall. Somtimes, retrospective studies are referred to as "chart reviews." Advantages to retrospective studies include lower cost, faster completion and use of records that already exist. However, there are significant disadvantages, including incomplete or inaccurate medical records, biased results due to lack of randomization or blinding, confounding variables and difficulty establishing cause and effect (Table 2).

The best use of a retrospective study is to generate hypothesis that can be tested in a prospective fashion. Unfortunately, the majority of studies in veterinary medicine are retrospective.

Table 3: Types of bias
Bias can be a problem in both retro-spective and prospective studies, but is more problematic in retrospective studies (Table 3).

Retrospective studies often suffer from recall bias, selection bias and observer bias. In retrospective studies, the information in the medical records was entered without any knowledge of a future retrospective study. This means that the medical records may be missing important data that the study conclusions will be based on.

For example, if a study is looking at tumor size done prior to the treatment of interest, the size of the tumor may not be in the record or the measurements may be estimated. Although the clinician can be asked if they can remember the tumor size, it is not likely to be an accurate measurement due to the length of time elapsed. (This is an example of recall bias.)

Retrospective studies are not randomized, so they often contain selection bias. Inadvertent selection bias occurs when the treatment is chosen based on prognostic factors, cost, treatment scheduling, etc. This results in groups that differ from each other in significant ways.

For example, patients may have received treatment only if they had certain negative prognostic factors, so it is not accurate to compare them to patients that did not receive treatment.

If there is not a contemporary control group, then historical controls may be substituted. Historical controls can introduce bias. For example, if there is an improvement in early detection, the historical controls may have had more advanced disease compared to the study population.

Prospective studies

Prospective studies are preferred over retrospective studies because the results are less likely to be influenced by bias and confounding factors.

In a prospective study, the data collection and the events of interest do not occur until after the patients are enrolled on the study, allowing for more consistent recording of data. Randomized prospective studies are preferred to avoid any selection bias on the part of the researcher or owner.

The goal of randomization is to create groups that are identical with the exception of the treatment being studied. If a study is blinded, it means that the owner does not know which group their pet is in. In some cases, a placebo is used if there is a group that does not receive treatment. In a double-blinded study, neither the owner nor researcher knows who has been assigned to which group. Blinding eliminates the potential for observer or experimenter's bias.

It has been said that it is possible to prove anything depending on how the statistics are manipulated. Without a background in statistics, it can be difficult to determine if the appropriate statistical analysis is being used in the study. There are a few basic considerations.

The significance of a finding is the likelihood that the finding is due to the treatment rather than by chance. If a finding is considered significant with a p value of .05, this means that the researcher is 95 percent confident that the treatment is responsible for the results.


Source: DVM360 MAGAZINE,
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