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).
Table 2: Retrospective studies
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.
Bias can be a problem in both retro-spective and prospective studies, but is more problematic in retrospective studies (Table
Table 3: Types of bias
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 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.