# Effect Size

In statistics, an effect size is a measure of the strength of a phenomenon (for example, the relationship between two variables in a statistical population) or a sample-based estimate of that quantity. An effect size calculated from data is a descriptive statistic that conveys the estimated magnitude of a relationship without making any statement about whether the apparent relationship in the data reflects a true relationship in the population. In that way, effect sizes complement inferential statistics such as p-values. Among other uses, effect size measures play an important role in meta-analysis studies that summarize findings from a specific area of research, and in statistical power analyses.

The concept of effect size appears already in everyday language. For example, a weight loss program may boast that it leads to an average weight loss of 30 pounds. In this case, 30 pounds is an indicator of the claimed effect size. Another example is that a tutoring program may claim that it raises school performance by one letter grade. This grade increase is the claimed effect size of the program. These are both examples of "absolute effect sizes", meaning that they convey the average difference between two groups without any discussion of the variability within the groups. For example, if the weight loss program results in an average loss of 30 pounds, it is possible that every participant loses exactly 30 pounds, or half the participants lose 60 pounds and half lose no weight at all.

Reporting effect sizes is considered good practice when presenting empirical research findings in many fields. The reporting of effect sizes facilitates the interpretation of the substantive, as opposed to the statistical, significance of a research result. Effect sizes are particularly prominent in social and medical research. Relative and absolute measures of effect size convey different information, and can be used complementarily. A prominent task force in the psychology research community expressed the following recommendation:

Always present effect sizes for primary outcomes...If the units of measurement are meaningful on a practical level (e.g., number of cigarettes smoked per day), then we usually prefer an unstandardized measure (regression coefficient or mean difference) to a standardized measure (r or d).

L. Wilkinson and APA Task Force on Statistical Inference (1999, p. 599)

Read more about Effect SizeConfidence Intervals By Means of Noncentrality Parameters, "Small", "medium", "large"

### Other articles related to "effect size, effect sizes, size, effect":

Discriminant Function Analysis - Effect Size
... Some suggest the use of eigenvalues as effect size measures, however, this is generally not supported ... Instead, the canonical correlation is the preferred measure of effect size ... Another popular measure of effect size is the percent of variance for each function ...
Alpha Blocker - Uses
... improvement in overall sleep quality (effect size, 1.6) and recurrent distressing dreams (effect size, 1.9) ... In each of the PTSD symptom clusters the effect size was medium to large 0.7 for reexperiencing or intrusion, 0.6 for avoidance and numbing, and 0.9 for hyperarousal ... in CGI-C scores (overall PTSD severity and function at endpoint) also reflected a large effect size (1.4) ...
Controversy - Opposition To The Dodo Bird Verdict
30 years ago a general equivalency of the effect sizes of psychotherapies may have seemed reasonable, that is not at all the case today ... In meta-analytic reviews, there is generally a larger effect size when CBT is used to treat anxiety disorders ... The larger the effect size in a meta-analysis, the more evidence there is to support a claim or therapy ...
Effect Size - "Small", "medium", "large"
... Some fields using effect sizes apply words such as "small", "medium" and "large" to the size of the effect ... Whether an effect size should be interpreted small, medium, or large depends on its substantial context and its operational definition ... Power analysis or sample size planning requires an assumed population parameter of effect sizes ...
Sympatholytic - Antiadrenergic - In Anxiety - Alpha Blockers
... Patients who were taking prazosin had a robust improvement in overall sleep quality (effect size, 1.6) and recurrent distressing dreams (effect size, 1.9) ... In each of the PTSD symptom clusters the effect size was medium to large 0.7 for reexperiencing or intrusion, and 0.6 for avoidance and numbing, and ... and function at endpoint) also reflected a large effect size (1.4) ...

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