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Measures of Effect Size in Medical Research

Learn how to interpret and compare medical research findings using measures of effect size and understand the importance of this statistical approach.
2023-03-15

Review of Measures of Effect Size in Medical Research

The use of measures of effect size is an important aspect of biostatistics and medical research. Effect size measures provide researchers with an estimate of the size of the treatment or intervention effect on the study population. This article will review the various measures of effect size that are used in medical research, and discuss the advantages and disadvantages of each.

What is an Effect Size?

Effect size is a statistical measure that describes the magnitude of the effect of an intervention or treatment on a population. It is commonly used to compare the effectiveness of different treatments, and to assess the strength of the relationship between two variables. Effect size measures are expressed as a numerical value, rather than as a percentage or ratio.

Types of Effect Sizes

The most commonly used measures of effect size in medical research are the standardized mean difference, Cohen's d, and the odds ratio.

Standardized Mean Difference

The standardized mean difference (SMD) is a measure of the difference between two means, expressed in standard deviation units. It is used to compare differences in means between two groups, and to compare the relative strengths of the relationships between two variables. The SMD is most commonly used when the two groups being compared are of different sizes, or when the distributions of the two variables being compared are not normal.

Cohen's d

Cohen's d is a measure of effect size that is based on the difference between two means, expressed in standard deviation units. It is used to compare the size of a treatment effect on a population, and to estimate the strength of the relationship between two variables. Cohen's d is most commonly used when the two groups being compared are of similar size, and when the distributions of the two variables being compared are normal.

Odds Ratio

The odds ratio (OR) is a measure of effect size that is based on the ratio of the odds of an event occurring in one group compared to the odds of it occurring in another group. It is used to compare the relative risk of an event occurring in one group compared to another group. The OR is most commonly used when the two groups being compared are of different sizes, and when the outcome being measured is binary (e.g. present/absent).

Advantages and Disadvantages of Effect Size Measures

Each of the measures of effect size discussed above has its own advantages and disadvantages.

Standardized Mean Difference

The main advantage of the SMD is that it is relatively easy to calculate and interpret, and is sensitive to small differences in means between groups. The main disadvantage is that it does not take into account the sizes of the two groups being compared.

Cohen's d

The main advantage of Cohen's d is that it is sensitive to small differences in means between groups, and takes into account the sizes of the two groups being compared. The main disadvantage is that it is more difficult to calculate and interpret than the SMD.

Odds Ratio

The main advantage of the OR is that it is easy to calculate and interpret, and takes into account the sizes of the two groups being compared. The main disadvantage is that it is not sensitive to small differences in means between groups.

Conclusion

In conclusion, the choice of measure of effect size depends on the type of data being analyzed, the size of the groups being compared, and the type of outcome being measured. Each measure of effect size has its own advantages and disadvantages, and researchers should carefully consider which one is most appropriate for their study.

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